wayland: Fix an offload cornercase

Treat textures with background as opaque for the purposes of
offloading.

gdk/wayland/gdksubsurface-wayland.c

@@ -450,7 +450,8 @@ gdk_wayland_subsurface_attach (GdkSubsurface         *sub,
                          self);
     }
   else if (!will_be_above &&
-           gdk_memory_format_alpha (gdk_texture_get_format (texture)) != GDK_MEMORY_ALPHA_OPAQUE)
+           gdk_memory_format_alpha (gdk_texture_get_format (texture)) != GDK_MEMORY_ALPHA_OPAQUE &&
+           !has_background)
     {
       GDK_DISPLAY_DEBUG (gdk_surface_get_display (sub->parent), OFFLOAD,
                          "Cannot offload non-opaque %dx%d texture below, hiding subsurface %p",