Multi window support

[Yamakaky]

When detecting window resize events, it doesn't check which window is resized. When multiple windows are used, events from other windows will affect egui. The fix is to check like this: https://github.com/Yamakaky/gyromouse/blob/gui/src/backend/sdl/overlay.rs#L221-L225.

https://github.com/ArjunNair/egui_sdl2_gl/blob/main/src/lib.rs#L54-L57

[ArjunNair]

Good point! I'll integrate that fix asap.

[ArjunNair]

I've added the suggested fixes. Would appreciate it you can confirm they look good.

[Yamakaky]

Just found out that we have the same problem for keyboard and mouse events. Fix: Yamakaky/gyromouse@55b26af

[ArjunNair]

Thanks. I've pushed changes to check for window id across events.

[Yamakaky]

Sgtm!