Implement weston_view_is_opaque()
I tried to implement weston_view_is_opaque() as described in #126 (closed).
The first patch adds a bool is_opaque property to weston_surface to indicate that the attached buffer has no alpha channel or the solid color is non-transparent. It is set by weston_surface_set_color (in common code) and by weston_surface_attach (per renderer).
The second patch uses that as well as the view's alpha value and opaque region to determine view opacity in a specified region.
The third patch uses weston_view_is_opaque() in the DRM compositor.