Custom cursor tracking issue

Custom cursors are still missing some refinements.

Known Issues

• On Web, correctly handle multiple windows waiting on the same cursor: #3321.
• On Wayland, account for alpha: #3360.
• Figure out how to prevent CustomCursors to outlive the event loop. This is similar to #3317 and #3258.

Improvements

• Add back Window::set_cursor_icon() as a deprecated method: #3329.
• Merge icons and cursors in one type and one method: https://github.com/rust-windowing/winit/pull/3308.
• Add a cursor getter/setter on WindowBuilder: #3319.
• Add a cursor getter on Window.
• Make sure implementations check if the new cursor being set isn't the same already set.
  • MacOS: #3324.
  • Wayland
  • Web: #3321.
  • Windows
  • X11
• Return a Result at CustomCursorBuilder::build(), AFAIK only Windows needs it.
• On Windows, extension method to load from a resource, like in Icon::from_resource().
• On Web, async fn build() extension method to wait until the cursor has successfully finished loading: #3384.

Caching

• MacOS: #3291
• Wayland
• Web: #3276
• Windows: #3293
• X11: #3366

Animation

• Web: #3384
• Windows: support ANI files (loaded from memory or otherwise).
• MacOS: Use NSTimer? (idea from Firefox)
• X11: XcursorAnimate

Start at the custom cursor entry points, including CustomCursorBuilder::build and Window, then compare the Wayland, Windows, and X11 implementations against the unchecked items. Choose one specific checklist item before investigating the relevant platform code; done means that item is implemented and its behavior is covered by the project’s existing checks.