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.