dri3: Wait for all pending swapbuffers to be scheduled before touching the front

dri3: Wait for all pending swapbuffers to be scheduled before touching the front

This implements a wait for glXWaitGL, glXCopySubBuffer, dri flush_front and
creation of fake front until all pending SwapBuffers have been committed to
hardware. Among other things this fixes piglit glx-copy-sub-buffers on dri3.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Cc: <mesa-stable@lists.freedesktop.org>