Support all wgpu surface targets

[brianreavis]

Objective

This PR refactors window handles / surface creation to support all the surface targets that wgpu supports (defined here) instead of just the subset that can be represented by raw-window-handle.

Although not supported out-of-the-box, the primary point for doing this is to unlock the ability to run Bevy entirely off the main thread on iOS and MacOS via wgpu’s SurfaceTargetUnsafe::CoreAnimationLayer.

Solution

Introduce a SurfaceTargetSource component on window entities that's very similar to RawHandleWrapper. It hands out a wgpu::SurfaceTarget or wgpu::SurfaceTargetUnsafe instead of a raw-window-handle handle.

Could this have been achieved by adding CoreAnimationLayer to raw-window-handle? Yes, but then that crate would need a new release and wgpu would need updated. The approach in this PR ensures Bevy supports all that wgpu supports without the potential for future drift.

Although it's now unused, RawHandleWrapper was left in place for anyone implementing custom renderers that aren't necessarily wgpu-based.

Showcase

struct CoreAnimationLayerHandle(*mut std::ffi::c_void);

unsafe impl Send for CoreAnimationLayerHandle {}
unsafe impl Sync for CoreAnimationLayerHandle {}

impl HasSurfaceTarget for CoreAnimationLayerHandle {
    unsafe fn surface_target(&self) -> Option<SurfaceTargetWrapper> {
        Some(SurfaceTargetWrapper::SurfaceTargetUnsafe(
            SurfaceTargetUnsafe::CoreAnimationLayer(self.0),
        ))
    }
}

// The basic idea:

let ca_layer_handle = CoreAnimationLayerHandle(ptr);
std::thread::spawn(move || {
    // (Bevy initialization goes here)
    // Spawn primary window
    let thread_constraint = SurfaceTargetThreadConstraint::None;
    let surface_target_source = SurfaceTargetSource::new(thread_constraint, ca_layer_handle);
    let primary_window = app
        .world_mut()
        .spawn((Window { ... }, PrimaryWindow, surface_target_source))
        .id();
    // (Custom render loop goes here)
});

Testing

• I tried to compartmentalize unsafe / minimize unsoundness, but could use another set of eyes on it.
• I tested the following configurations:
  • Run Bevy fully on a background thread on MacOS/iOS via SurfaceTargetUnsafe::CoreAnimationLayer (note: this isn’t supported out of the box in Bevy, but with this PR, it can be achieved with custom initialization similar to what’s shown in the Showcase section above)
  • Run on UiKit / AppKit on main thread via custom implementation
  • Run normal Bevy with bevy_winit on MacOS

Migration Guide

• If you are not using bevy_winit but are using bevy_render (uncommon), you must insert a SurfaceTargetSource component on window entities. It's nearly identical to RawHandleWrapper, but supports additional surface targets that cannot be represented by raw-window-handle.
• RawHandleWrapperHolder has been removed. Use RawHandleWrapper to get a handle instead.