compositor-drm: Determine opaqueness of a surface correctly
compositor-drm: Determine opaqueness of a surface correctly

We subtract the screen space opaque region from the transformed surface
region.  That's going to fail for when we're scaling up a surface, since
the surface bounding box for the scaled up surface is going to be bigger
than the opaque region.  Instead, subtract the surface-space opaque region
from a 0,0 - width,height region and see if that's empty.

eyolfson/weston,Fantu/compositor-spice,Fantu/compositor-spice,krezovic/weston,Fantu/compositor-spice,Tarnyko/weston-xdg_surface_present,xorgy/weston,giucam/weston,Gnurou/weston,mchalupa/weston,jonnylamb/weston,sir-murray/weston,giucam/weston,Gnurou/weston,jonnylamb/weston,xorgy/weston,Tarnyko/weston-xdg_surface_present,eyolfson/weston,kwm81/weston,kwm81/weston,udoprog/weston,krezovic/weston,udoprog/weston,sir-murray/weston,mchalupa/weston