Xwayland should set opaque region for non-ARGB children of ARGB toplevel windows

When a toplevel window using the ARGB visual has children using non-ARGB visuals, Xwayland should call wl_surface_set_opaque_region for the region covered by those children, to let the Wayland compositor know which parts of the surface are effectively opaque, allowing the compositor to perform corresponding optimizations.

The immediate motivation is mutter 44 always using the ARGB visual for server-side window decorations, which means most X windows are treated as fully potentially translucent. (Right now this can even prevent direct scanout of fullscreen windows, though that might get mitigated on the mutter side).