virgl: do color-conversion during when mapping transfer

virgl: do color-conversion during when mapping transfer

When running on OpenGL ES, we can't just map any format for reading,
because of limitations on glReadPixels. So let's fall back to the
blit code-path, and translate the pixels to the correct format in the
end.

This fixes the remaining failures of KHR-GL32.packed_pixels.* apart
from the sRGB tests.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>