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

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