ebiten: add an API to run on a UI thread

[hajimehoshi]

Operating System

• Windows
• macOS
• Linux
• FreeBSD
• OpenBSD
• Android
• iOS
• Nintendo Switch
• PlayStation 5
• Xbox
• Web Browsers

What feature would you like to be added?

I propose to have an API to run a given function on a main (UI) thread:

// RunOnUIThread runs the function f on the UI thread.
func RunOnUIThread(f func())

Why is this needed?

We want a safe area to access native APIs. Ebitengine runs its application on the main (UI) thread, but once RunGame starts, there is no way to run something from the thread.

An actual use case is to modify a native window, for example. See also #3381 (comment)