Panic: Non-antialiased, hairline stroke, full width rectangle

Nobody has claimed this yet.

Hi! I'm getting a panic similar to #20 and #47 with the following code on the latest master:

```rust
use tiny_skia::*;

fn main() {
let mut pixmap = Pixmap::new(100, 100).unwrap();

let mut path_builder = PathBuilder::new();

let rect = Rect::from_xywh(0.0, 0.0, 100.0, 100.0).unwrap();
path_builder.push_rect(rect);

let path = path_builder.finish().unwrap();

let mut paint = Paint::default();
paint.anti_alias = false;

let mut stroke = Stroke::default();
stroke.width = 0.0;

pixmap.stroke_path(&path, &paint, &stroke, Transform::identity(), None);
}
```
Panic backtrace:
```
thread 'main' panicked at 'range end index 10001 out of range for slice of length 10000', /home/kosa/.local/share/cargo/git/checkouts/tiny-skia-ff2529ae49675170/d2b7abb/src/pipeline/blitter.rs:325:21
stack backtrace:
0: 0x5630a5a586c1 - std::backtrace_rs::backtrace::libunwind::trace::h5554868a335b687a
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
1: 0x5630a5a586c1 - std::backtrace_rs::backtrace::trace_unsynchronized::hfd24c2b1bc872c7f
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
2: 0x5630a5a586c1 - std::sys_common::backtrace::_print_fmt::hb40d4835c8544bc2
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/sys_common/backtrace.rs:65:5
3: 0x5630a5a586c1 - ::fmt::h3d8750c5a9a530aa
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/sys_common/backtrace.rs:44:22
4: 0x5630a5a7439f - core::fmt::rt::Argument::fmt::h4d7415a32daf52e6
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/core/src/fmt/rt.rs:138:9
5: 0x5630a5a7439f - core::fmt::write::h15119af0af627cb4
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/core/src/fmt/mod.rs:1094:21
6: 0x5630a5a56787 - std::io::Write::write_fmt::hc629dbaef083aeea
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/io/mod.rs:1714:15
7: 0x5630a5a584d5 - std::sys_common::backtrace::_print::he9d2145ade770793
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/sys_common/backtrace.rs:47:5
8: 0x5630a5a584d5 - std::sys_common::backtrace::print::h026f51a088c21258
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/sys_common/backtrace.rs:34:9
9: 0x5630a5a598b3 - std::panicking::default_hook::{{closure}}::h34f469ea2c03adc7
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/panicking.rs:269:22
10: 0x5630a5a59644 - std::panicking::default_hook::hfd3e1ef4c97a68b4
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/panicking.rs:288:9
11: 0x5630a5a59e39 - std::panicking::rust_panic_with_hook::hc3be44b7757db76f
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/panicking.rs:705:13
12: 0x5630a5a59d37 - std::panicking::begin_panic_handler::{{closure}}::ha8f95c8316903adf
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/panicking.rs:597:13
13: 0x5630a5a58af6 - std::sys_common::backtrace::__rust_end_short_backtrace::h50cf4d5ceed3b7ca
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/sys_common/backtrace.rs:151:18
14: 0x5630a5a59a82 - rust_begin_unwind
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/panicking.rs:593:5
15: 0x5630a595eea3 - core::panicking::panic_fmt::hb952c057152cc209
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/core/src/panicking.rs:67:14
16: 0x5630a595f472 - core::slice::index::slice_end_index_len_fail_rt::h11d36dbe3c608f5c
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/core/src/slice/index.rs:76:5
17: 0x5630a595f472 - core::slice::index::slice_end_index_len_fail::hd44b49b92f227a9f
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/core/src/slice/index.rs:68:9
18: 0x5630a598d95f - as core::slice::index::SliceIndex<[T]>>::index_mut::h9535f765aa202fac
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/core/src/slice/index.rs:419:13
19: 0x5630a5a11deb - core::slice::index:: for [T]>::index_mut::h40da4673d4351703
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/core/src/slice/index.rs:29:9
20: 0x5630a59693ff - ::blit_rect::h10aa2c5f5a8a1145
at /home/kosa/.local/share/cargo/git/checkouts/tiny-skia-ff2529ae49675170/d2b7abb/src/pipeline/blitter.rs:325:21
21: 0x5630a59689ca - ::blit_h::h900ce4906ed2fae0
at /home/kosa/.local/share/cargo/git/checkouts/tiny-skia-ff2529ae49675170/d2b7abb/src/pipeline/blitter.rs:233:9
22: 0x5630a5a0a810 - tiny_skia::scan::hairline::hair_line_rgn::hb4d2b97ebae2968c
at /home/kosa/.local/share/cargo/git/checkouts/tiny-skia-ff2529ae49675170/d2b7abb/src/scan/hairline.rs:141:21
23: 0x5630a5a0b33f - tiny_skia::scan::hairline::stroke_path_impl::h636291936c1eb32f
at /home/kosa/.local/share/cargo/git/checkouts/tiny-skia-ff2529ae49675170/d2b7abb/src/scan/hairline.rs:224:17
24: 0x5630a5a0a063 - tiny_skia::scan::hairline::stroke_path::h7a059ef1df4ac2e3
at /home/kosa/.local/share/cargo/git/checkouts/tiny-skia-ff2529ae49675170/d2b7abb/src/scan/hairline.rs:36:5
25: 0x5630a598d5a2 - tiny_skia::painter::::stroke_hairline::h346e8764c597db6b
at /home/kosa/.local/share/cargo/git/checkouts/tiny-skia-ff2529ae49675170/d2b7abb/src/painter.rs:462:13
26: 0x5630a598cdf6 - tiny_skia::painter::::stroke_path::h9ffd16c046aac18a
at /home/kosa/.local/share/cargo/git/checkouts/tiny-skia-ff2529ae49675170/d2b7abb/src/painter.rs:430:21
27: 0x5630a598ad70 - tiny_skia::painter::::stroke_path::h09e80b5072064ede
at /home/kosa/.local/share/cargo/git/checkouts/tiny-skia-ff2529ae49675170/d2b7abb/src/painter.rs:143:9
28: 0x5630a595fa1f - testing::main::hd5feaac4d9e7398b
at /home/kosa/Documents/code/rust/testing/src/main.rs:19:5
29: 0x5630a595fb2b - core::ops::function::FnOnce::call_once::hdab0e40803267661
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/core/src/ops/function.rs:250:5
30: 0x5630a595fbae - std::sys_common::backtrace::__rust_begin_short_backtrace::h02781e58294858a3
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/sys_common/backtrace.rs:135:18
31: 0x5630a595f771 - std::rt::lang_start::{{closure}}::h84b2f74c486ba6a1
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/rt.rs:166:18
32: 0x5630a5a547fb - core::ops::function::impls:: for &F>::call_once::h09286b5638482068
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/core/src/ops/function.rs:284:13
33: 0x5630a5a547fb - std::panicking::try::do_call::hc4886f4fd1a5066c
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/panicking.rs:500:40
34: 0x5630a5a547fb - std::panicking::try::hd44a65a82ab950ab
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/panicking.rs:464:19
35: 0x5630a5a547fb - std::panic::catch_unwind::h0ee9aa0a1712a08d
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/panic.rs:142:14
36: 0x5630a5a547fb - std::rt::lang_start_internal::{{closure}}::h1f3410f2df20a4d3
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/rt.rs:148:48
37: 0x5630a5a547fb - std::panicking::try::do_call::h8900088bbeaeb111
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/panicking.rs:500:40
38: 0x5630a5a547fb - std::panicking::try::hbd6d94ead609e919
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/panicking.rs:464:19
39: 0x5630a5a547fb - std::panic::catch_unwind::h26d2b6e104bf4814
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/panic.rs:142:14
40: 0x5630a5a547fb - std::rt::lang_start_internal::h71c5256da3e502b7
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/rt.rs:148:20
41: 0x5630a595f74a - std::rt::lang_start::ha1b2d54c9c84c075
at /rustc/36fb58e433c782e27dd41034284e157cf86d587f/library/std/src/rt.rs:165:17
42: 0x5630a595fabe - main
43: 0x7f5cbf8cd850 -
44: 0x7f5cbf8cd90a - __libc_start_main
45: 0x5630a595f635 - _start
46: 0x0 -
```

Conditions:
1. Non-antialiased
2. Hairline stroke
3. The width of the `Rect` is the width of the `Canvas`

Changing either one of the three will run without a panic.

Reproduce the panic with the Rust snippet, then inspect src/pipeline/blitter.rs:325 and the hairline path in src/scan/hairline.rs:141 and :224. Add a regression test covering a non-antialiased, zero-width stroke over a full-width rectangle, and verify that it completes without panicking.

Written by the indexing model from the issue text.