compositor: set the opaque region for some views with transform
compositor: set the opaque region for some views with transform

If the transform on a view is only a translation we can trivially
set the opaque region for it so to optimize the rendering.
Reviewed-by: Jonas Ådahl <67dcd8a17e758192e0cbb577ce1d62cc7da6cf49@gmail.com>

sir-murray/weston,sir-murray/weston