Don't taint the application framebuffer with overlays

Currently applications doing partial redraws are broken, as overlays that draw on top of the application's framebuffer affect its state. For actual "first class partial redraw support" this needs to be changed so the app's framebuffer content remains stable regardless of overlays.

It's known and well-discussed, just splitting it from #77 (closed) which got already closed.

!257 (merged) appears to have made some steps towards that, but it's not there yet.