We were supplying __DRI2_THROTTLE_SWAPBUFFER, rather than the obvious
choice of __DRI2_THROTTLE_COPYSUBBUFFER. This meant that we hit the
swap-based frame throttling. glXCopySubBuffer doesn't seem like it's
intended to be a frame boundary, so we'd like to avoid this throttling.
Tested-by: Michel Dänzer <mdaenzer@redhat.com> # DRI3 only
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>