Add first-party asset-based cursor icons to Bevy

[alice-i-cecile]

FYI, winit's existing set_cursor_icon method is inadequate for games: it only supports a fixed, unstyled list and relies on OS support. In games, you often need new options (a mining pick! an axe! a sword!), and virtually always want custom assets.

This isn't something winit can fix, since it doesn't come with an asset / rendering solution. Instead, we should build our own cursor icon solution, with a "native" variant that just uses the OS ones provided by winit.

Originally posted by @alice-i-cecile in #9538 (reply in thread)

[alice-i-cecile]

The first PR for this should be quite simple: just create a UI element from an image, and keep it synced with the mouse while disabling the built in cursor.

Probably have a CursorBundle or something, and then just let users manually swap the image handle as needed.

Picking-integrated reactivity can come later.

[aevyrie]

We actually need winit support to do this correctly. Relying on the rendered window to draw a cursor is never going to have the same low latency as the OS cursor.

rust-windowing/winit#3005

I've noticed other games with custom cursors have properties that prove this:

• during game streaming, the custom cursor will become visible before the rendered output is visible
• the cursor can leave the frame of the window when the cursor is positioned in the bottom right
• the latency and update rate is perfect even when the game is not running amazingly well

Regardless, I agree an asset-based solution would be nice, but I wanted to point out we do need to use OS APIs to do this without jank.

[aevyrie]

More proof this is possible, navigate to this page and hover over the demo to get the rainbow icon.

https://webdesign.tutsplus.com/how-to-create-a-custom-mouse-cursor-with-css-or-javascript--cms-106724t

Scroll up so the demo is at the bottom edge of the screen. You will be able to see the custom cursor image hanging outside the bounds of the browser window. Additionally, the latency is perfect.

[alice-i-cecile]

Definitely agree that we should hook into the OS layer.

Unsure if there's a sensible way to do this with winit itself though 🤔 It's not asset or graphics aware. Same sort of problem we ran into with window icons.

[aevyrie]

The winit API should just accept image bytes for the Custom variant. We just need a layer of indirection on the current Window cursor setting that accepts an asset handle once winit supports the feature, and a plugin that updates it with the data in the Handle<Image>.

[Friz64]

rust-windowing/winit#3218 has just been merged 🎉

[djeedai]

Came to say the same :) Is there a tracking issue for upgrading winit itself, or a process, or someone usually handling it? Note that a winit version with that feature did not release yet, but that should happen soon I guess.

[alice-i-cecile]

#10702 just bumped the winit version to the latest release :) For the next release, whoever wants to can claim it. Just split apart the "minimal migration" from the "use new feature" for easier reviews.

[eero-lehtinen]

Winit maintainers said that the changes won't be released in a patch version, so we might need to wait for a while. They also want to bake the feature a little bit more.