grabs a port. Once a client grabs a port, only that client can make video
requests to that port.
If the time is before the current port time, the XvGrabPort request is
ignored and XvInvalidTime is returned. If the port is already grabbed by
another client, XvAlreadyGrabbed is returned. Otherwise it returns a
status of Success. The port time is updated when the following
requests are processed:
If the port is actively processing video for another client, the
video is preempted, and an XvVideoNotify event with detail XvPreempted
is generated for its drawable.