(core+) multi-display/modeswitch improvements

The dynamic multiple output displays are closer to working
properly now for egl-dri (on my limited number of test
devices at least, it is not a trivial interface..)

Though there are several more advanced experiments and features
to squeeze in there, the basics seem to work except for the
following outstanding issues:

 The per-display projection matrices seem to be calculated
 correctly but the result isn't correct (use the interactive
 scan test with modesetting to see)

 The map_video_display only considers WORLDID and ignores
 source object texture coordinates, this should be quick / trivial
 to get sorted.