Segfault when run from child thread.

Hello! Thank you for building this, first of all! I've had alright success running this from the main thread, but it seems to die when run from a child. Here's the segfault:

```
1 0x7fdcc656c247 /usr/lib/x86_64-linux-gnu/libjavascriptcoregtk-4.0.so.18(WTFCrash+0x17) [0x7fdcc656c247]
2 0x7fdcc7ced999 /usr/lib/x86_64-linux-gnu/libwebkit2gtk-4.0.so.37(+0x67d999) [0x7fdcc7ced999]
3 0x7fdcc7d31d9a /usr/lib/x86_64-linux-gnu/libwebkit2gtk-4.0.so.37(+0x6c1d9a) [0x7fdcc7d31d9a]
4 0x7fdcc7d32219 /usr/lib/x86_64-linux-gnu/libwebkit2gtk-4.0.so.37(+0x6c2219) [0x7fdcc7d32219]
5 0x7fdcc7faf0d0 /usr/lib/x86_64-linux-gnu/libwebkit2gtk-4.0.so.37(+0x93f0d0) [0x7fdcc7faf0d0]
6 0x7fdcc6833e72 /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0(g_object_unref+0x1a2) [0x7fdcc6833e72]
7 0x7fdcc48f0041 /lib/x86_64-linux-gnu/libc.so.6(+0x43041) [0x7fdcc48f0041]
8 0x7fdcc48f013a /lib/x86_64-linux-gnu/libc.so.6(+0x4313a) [0x7fdcc48f013a]
9 0x7fdcc48ceb9e /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xee) [0x7fdcc48ceb9e]
10 0x55777992389a target/debug/rust_ksl_uploader(+0x889a) [0x55777992389a]
Segmentation fault (core dumped)
```

And here's the minimal code to reproduce:

```rust
extern crate web_view;

use web_view::*;
use std::thread::spawn;

const INDEX: &str = r#"

This is a test.

"#;

fn main() {
let size = (800, 800);
let resizable = true;
let debug = true;
let init_cb = |_webview| {};
let frontend_cb = |_webview: &mut _, _arg: &_, _userdata: &mut _| {};
let userdata = ();
let view = spawn(move || {
run("test thread", Content::Html(INDEX), Some(size), resizable, debug, init_cb, frontend_cb, userdata);
});

view.join().expect("Join Error");
}
```