Implement WinRT support in SDL2 raw-window-handle shim

[nathanvoglsam]

Now that #1182 is merged the crate is up to date with the latest raw-window-handle release. This release contains the results of my PR from February which adds support for WinRT (UWP) to the crate. raw-window-handle now exposes a WinRt variant which contains an opaque c_void pointer to what is expected to be a CoreWindow*. SDL2 provides the same mechanism with the SDL_SYSWM_WINRT enum variant and the winrt field on SDL_SysWMinfo, which is what I modeled the WinRt variant in raw-window-handle after.

As a result, this PR includes the additions necessary to shim the CoreWindow pointer from SDL2 to raw-window-handle, allowing the use of this crate on UWP.

The changes simply adds the required match branch to output the RawWindowHandle::WinRt variant, as well as modifying the union definition so we can shim it from SDL2. SDL2 already handles the UWP pain for us.

Here's egui rendered with DirectX 12 in a UWP app on Windows 10.

I would include it running on my Xbox Series S too if I hadn't loaned it to someone.

The source code for my implementation of the UWP app is here https://github.com/nathanvoglsam/aleph.

[Cobrand]

Thanks!