Refactor offscreens to not recreate them from scratch every frame

[YaLTeR]

Currently we use offscreens only for animations (open, close, resize) which are short and therefore not too bad to redraw every frame. However, I want to use offscreens for longer things like interactive move (make window semitransparent), or for more things at once (the tab fade animations should use offscreens ideally). For this we should do offscreens better: keep the texture around, along with a damage tracker, and only redraw the changes.

This should also hopefully help with issues like #1138.