Convert renderer debug bindings into debug scopes
Weston has some debug key bindings in the renderers that toggle certain rendering debugging modes, including the triangle fan to see the primitive edges and the green tint to identify composite bypass. Activating these via the keyboard combination is slightly awkward.
We could add a debug scope for each toggle. As long as the debug scope has subscribers, keep the debug function on. That way one could use
weston-debug to enable these modes, and quitting it will restore to normal.