actor: Use the real opacity when clearing the stage
actor: Use the real opacity when clearing the stage

The paint opacity for a top level is always overridden to be the full
value, since it's a composited value and we want to paint our scene.

When clearing the stage framebuffer, though, we want to use the actual
opacity, if ClutterStage:use-alpha is set.

Distrotech/clutter,Distrotech/clutter,jigpu/clutter,GNOME/clutter,jigpu/clutter,Distrotech/clutter,GNOME/clutter,jigpu/clutter,jigpu/clutter,GNOME/clutter,GNOME/clutter,Distrotech/clutter,Distrotech/clutter,GNOME/clutter,Distrotech/clutter,jigpu/clutter,jigpu/clutter