compositor: add WESTON_COMPOSITOR_OFFSCREEN state

This state is used when the user switches the vt. It turns of rendering
and frame events, but doesn't set the DPMS state to off.

As a part of this change, also turn off the idle timer when entering
the SLEEPING or OFFSCREEN states, which fixes
https://bugs.freedesktop.org/show_bug.cgi?id=61910 (rpi backend
untested).
5 files changed