renderer-gl: only flush damage for surfaces used in the next output repaint

Stop flushing surfaces that are put on an overlay plane on the output to
be repainted next, but that have to be painted into another output's
primary_plane.

Now that each output has its own primary_plane, and flush_damage() knows
the output that is going to be repainted, texture_used can be limited to
surfaces that will actually be used by the renderer during the following
repaint_output().

Always upload when called from gl_renderer_surface_copy_content() or
gl_renderer_create_surface() with the output parameter set to NULL.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>