Panic : ClipMask::set_path (debug_assert!)

Nobody has claimed this yet.

The code below panics.
Latest version : tiny-skia = "0.6.6"

The panic come from the a `debug_assert!` in `src/scan/path.rs`

```rust
use tiny_skia::*;

fn main() {
// special value
let v = -6.40969;

let clip_path = {
let mut pb = PathBuilder::new();
pb.push_circle(v, v, v);
// special values
pb.push_circle(823811.0, v, 824467.0);
pb.finish().unwrap()
};

// special values
let clip_path = clip_path.transform(Transform::from_row(v, 824060.0, 0.0, 0.0, 0.0, 0.0)).unwrap();

let mut clip_mask = ClipMask::new();
// panic here
clip_mask.set_path(500, 500, &clip_path, FillRule::EvenOdd, true);
}
```

Stacktrace :

```
thread 'main' panicked at 'assertion failed: edges[curr_idx].last_y >= curr_y as i32', ~/.cargo/registry/src/github.com-1ecc6299db9ec823/tiny-skia-0.6.6/src/scan/path.rs:178:13
stack backtrace:
0: rust_begin_unwind
at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/panicking.rs:584:5
1: core::panicking::panic_fmt
at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/core/src/panicking.rs:143:14
2: core::panicking::panic
at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/core/src/panicking.rs:48:5
3: tiny_skia::scan::path::walk_edges
at ~/.cargo/registry/src/github.com-1ecc6299db9ec823/tiny-skia-0.6.6/src/scan/path.rs:178:13
4: tiny_skia::scan::path::fill_path_impl
at ~/.cargo/registry/src/github.com-1ecc6299db9ec823/tiny-skia-0.6.6/src/scan/path.rs:155:5
5: tiny_skia::scan::path_aa::fill_path_impl
at ~/.cargo/registry/src/github.com-1ecc6299db9ec823/tiny-skia-0.6.6/src/scan/path_aa.rs:106:5
6: tiny_skia::scan::path_aa::fill_path
at ~/.cargo/registry/src/github.com-1ecc6299db9ec823/tiny-skia-0.6.6/src/scan/path_aa.rs:62:5
7: tiny_skia::clip::ClipMask::set_path
at ~/.cargo/registry/src/github.com-1ecc6299db9ec823/tiny-skia-0.6.6/src/clip.rs:94:13
```

Reproduce the provided example with tiny-skia 0.6.6, then inspect src/scan/path.rs around the debug_assert! in walk_edges and the callers in src/scan/path_aa.rs and src/clip.rs. Add regression coverage for the panic case and verify that the same input no longer panics while preserving the expected ClipMask behavior.

Written by the indexing model from the issue text.