Fix multi-threaded Wasm

[daxpedda]

When I tried actually using Softbuffer with #76 in a multi-threaded Wasm environment using transferControlToOffscreen() I found that Softbuffer actually fails on ImageData::new_with_u8_clamped_array(wasm_bindgen::Clamped(&bitmap), width.into()).

Apparently it is not allowed to create a ImageData from a SharedArrayBuffer. I only found a couple of Stackoverflow issues facing this (#1, #2) but no actual documentation on MDN or WHATWG.

This fixes it by copying the data to a Uint8Array first and then creating the ImageData from that. Which is an extra copy that can't be avoided when using multi-threaded Wasm. I hid it all behind #[cfg(target_feature = "atomics")] to make sure not to affect people not needing this.

This can be further optimized by keeping the Uint8Array and not having to recreate it from scratch again every time. I thought maybe we should wait for #65 before that, happy to add it now if desired.
This could be further optimized if we add an additional function for Web that uses a copy API instead of the currently owned buffer one.

To make this happen I had to do an import by hand because web-sys doesn't actually provide the constructors we need for ImageData.

I also added -Ctarget-feature=+atomics to the CI separately to the regular Wasm build.

See the wasm-bindgen documentation for more information on multi-threaded Wasm.

This can be run locally with RUSTFLAGS=-Ctarget-feature=+atomics,+bulk-memory cargo +nightly run-wasm --example winit -Zbuild-std=panic_abort,std.

[daxpedda]

I am assuming that the CI test I added is not executed because of some security settings on the repo.

[daxpedda]

Now that we decided that #76 should not be merged until wasm-bindgen releases a new version with the imports we needed (#76 (comment)), I rebased this PR onto master.

To address any potential concerns (considering @madsmtm didn't want to use custom imports but wait for wasm-bindgen/web-sys), the imports I introduced here won't make it into the wasm-bindgen ecosystem. This is because they already have their own custom variant of Uint8ClampedArray which really doesn't work the same way other wasm-bindgen imports work. This would require a breaking change on their side, not really something that will happen anytime soon.

[daxpedda]

CI fails because the new version of raw-window-handle now requires Rust v1.64, but our CI runs on v1.60:
https://docs.rs/crate/raw-window-handle/0.5.1/source/Cargo.toml

[notgull]

If that's the blessed way of doing it, then I suppose everything else looks good enough.

[notgull]

Try rebasing on top of the new master

[daxpedda]

Done!

[daxpedda]

Rebased after #65.

[notgull]

This works on my machine under run-wasm, so I'll just merge it.