libweston: Skip views without a layer assignment in output_mask calculations

Surface views that are not assigned to a layer are not going to be rendered, and thus should not participate in determining the outputs the surface is on.

There are other view properties that may determine if the view should be considered in output_mask calculations, e.g., is_mapped, but checking for this currently breaks tests. Such additional checks are left for future fixes or reworkings of the view infrastructure.

Fixes #646 (closed)

Signed-off-by: Alexandros Frantzis alexandros.frantzis@collabora.com