Restrict 1x1 pixmap filling optimization to GXcopy

This restricts an optimization whereby the filling of 1x1 pixmaps
went around the driver-provided function to cases where the
source color is meant to be directly copied to the destination,
as opposed to other operations which should produce different
destination values than just the foreground color.

Signed-off-by: George Matsumura <gmmatsumura01@bvsd.org>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
3 jobs for onepixel_fill_fix in 5 minutes and 25 seconds