compositor: turn weston_view::transform.opaque into masked

compositor: turn weston_view::transform.opaque into masked

Turns out there were no users of weston_view::transform.opaque,
everything was already using transform.masked_opaque. Therefore
repurpose transform.opaque as masked_opaque, and remove masked_opaque
member.

Now this opaque region in global coordinates is clipped by the layer
mask, if set. There are no cases where you would need the opaque region
without the effect of layer mask.

Also add a note in compositor.h, that changing view's layer counts as
changing geometry, which requires calling weston_view_geometry_dirty()
to let all derived state update.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-By: Giulio Camuffo <giuliocamuffo@gmail.com>