Add RGBA cursor icon support on Windows

[rofferom]

This MR aims to be a shorter version of #1617. Note this is currently a draft to discuss if the global approach is valid from the maintainers point of view.

The goal is to have a lower level API (compared to #1617) that only provides a RGBA bitmap, its size, and the cursor hotspot, with empty implementation easy to add for other platforms.

However, I have some remarks about my patch

• I'm quite noobish in Rust, and it is my first contribution attempt to winit. I'm pretty sure my patch is a mess :)
• The naming is dirty, but I'm confident we can find something better
• I can also provide a X11 implementation if requested
• The Windows implementation is basically a port of SDL2 code
• The whole CursorRgba content must be confirmed

Checklist

• [ ] Tested on all platforms changed
• [ ] Compilation warnings were addressed
• [ ] cargo fmt has been run on this branch
• [ ] cargo doc builds successfully
• [ ] Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• [ ] Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• [ ] Created or updated an example program if it would help users understand this functionality
• [ ] Updated feature matrix, if new features were added or implemented