FrameGrabber: Derive frame timeouts from the expected frame rate

Tests that limit the capture frame rate waited for the full global
timeout before giving up on a missing frame.

This patch lets FrameGrabber::durationBetweenFrames() and
waitAndTake*() take the expected frame rate, so they time out shortly
after a frame is overdue. The first frame keeps the global timeout.

Pick-to: 6.12
Change-Id: I5a0483bf8c445fc2fe260e608ef7f2e3ac209b84
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>
Reviewed-by: Tim Blechmann <tim.blechmann@qt.io>