SDL2/3 Platform backend

[flukejones]

Feature Description

Winit (wayland) currently has an issue where the window seems to never show, no matter what settings are used if started via Steam in a gamescope session (aka, steamos). For anyone attempting to add UI to SteamOS this becomes an issue as XWayland is not all that great either.

For my specific use-case a platform implementation that uses SDL2 or SDL3 would solve a whole host of issues, and provide x11, wayland, and kms backends, plus gamepad inputs, touches, and a few other extras.

Being able to use the SDL event loop would also allow another class of games to use slint for UI (such as how egui is used).

[tronical]

That seems like a good idea to me.

[flukejones]

The related issue I filed in winit: rust-windowing/winit#4152

[ogoffart]

Note that this should be implementable using our slint::platform::Platform without the need to have to implement that in Slint itself.