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
Status Job ID Name Coverage
  Docker Image
passed #1473233
debian-testing

00:01:02

 
  Build And Test
passed #1473234
autotools-build-and-test

00:04:23

passed #1473235
meson-build-and-test

00:03:15