[branch-merge] Support multi-head renderering

This merges the "seat-rework" branch to master.

Instead of talking to /dev/fb, we now use libdrm
to set up the drawing area.  This gives us the
functionality we need for querying the user's
monitor layout.

There are cases where /dev/fb works when the new
interfaces don't.  For those cases we fall back
to /dev/fb.