weston_view::transform.boundingbox is made to include the layer mask,
which removes the need for masked_boundingbox.

The following were using boundingbox when they should have used
masked_boundingbox:
- drm_output_prepare_overlay_view() uses boundingbox to compute overlay
  position, source and destination coordinates.
- drm_assign_planes() uses boundingbox for view overlap checks.
- is_view_not_visible() uses boundingbox, but nothing will show outside
  the layer mask.
- weston_surface_assign_output() intersects boundingbox with output
  region to choose the primary output for a surface.
- weston_view_assign_output() intersects boundingbox with output region
  to pick the outputs the view is on.

This patch essentially changes all those cases to use the masked
boundingbox.

Therefore there are no cases which would need the boundingbox without
the layer mask, and we can convert boundingbox into masked and remove
the left-over member.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
[v2: don't move the decl of 'mask' in weston_view_update_transform]
Reviewed-By: Giulio Camuffo <giuliocamuffo@gmail.com>