st/dri: Perform MSAA downsampling for __DRI2_THROTTLE_COPYSUBBUFFER

st/dri: Perform MSAA downsampling for __DRI2_THROTTLE_COPYSUBBUFFER

glXCopySubBufferMESA copies data from the back buffer to the front,
so it needs to perform a MSAA downsampling operation just like
glXSwapBuffers would.

Currently, the CopySubBuffer implementations supply a throttle reason
of __DRI2_THROTTLE_SWAPBUFFERS, so they hit this path and work today.
But we'd like to avoid swapbuffer throttling in this case, so the next
patch will change that reason.

Tested-by: Michel Dänzer <mdaenzer@redhat.com> # DRI3 only
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>