gallium/st: Add a method to flush outstanding swapbuffers

gallium/st: Add a method to flush outstanding swapbuffers

Add a state tracker interface method to flush outstanding swapbuffers, and
add a call to it from the mesa state tracker during glFinish().
This doesn't strictly mean the outstanding swapbuffers have actually finished
executing but is sufficient for glFinish()
to be able to be used as a replacement for glXWaitGL().

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>