gl-renderer: move undo-premult to color_pipeline()

Now that we have the if-else ladder to call color_pipeline() only when
necessary, and since only color_pipeline() needs undo-premult, move
undo-premult into color_pipeline().

This is a small step towards improving code readability.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>