ebiten: An API to skip Draw when Update is not called

[hajimehoshi]

From the discussion at #1520 and #1595, we might want an API to skip Draw calls when Update is not called (especially when vsync is off). In most games, the state should be updated only in Update and then Draw doesn't have to be called when Update is not called. In this case, the last offscreen result can be reused.

As the side effect of skipping, FPS will no longer represent the number of Draw calls per second.

[hajimehoshi]

https://github.com/hajimehoshi/ebiten/discussions/2061

There are some confusions in Update and Draw. I'd like to confirm the current implementation of Update/Draw is good or not...