server/red_worker/video: maintain visible region and clip region for streams
server/red_worker/video: maintain visible region and clip region for streams

Differentiate between the clipping of the video stream, and the region
that currently displays fragments of the video stream (henceforth,
vis_region). The latter equals or contains the former one. For example,
let c1 be the clip area at time t1, and c2 be the clip area at time t2,
where t1 < t2. If c1 contains c2, and at least part of c1/c2, hasn't been
covered by a non-video images, vis_region will contain c2, and also the part
of c1/c2 that still displays fragments of the video.
When we consider if a stream should be "upgraded" (1), due to its area
being used by a rendering operation, or due to stopping the video, we
should take into account the vis_region, and not the clip region (next
patch: not upgrade by the last frame, but rather by vis_region).
This fix will be more necessary when sized frames are introduced (see the
following patches). Then, the vis_region might be larger
than the last frame, and contain it, more frequently than before.

(1) "upgrading a stream" stands for sending its last frame losslessly. Or more
precisely, lossless resending of all the currently displayed lossy areas, that were
sent as part of the stream.

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