WIP: stage: Hide the stage while monitors are off

This saves power by ensuring no new frames get rendered, and it also avoids asking the backend to flip frames to unresponsive monitors.

We used to do exactly this (by accident?) before f57ce725, so it's not really anything new.

Closes: #735 (closed) (though only really as a workaround, which is the same as pre- f57ce725)