* Tracking: bounded crop size, Roto chain span and resection copies
A stored pre-tracking blur or search area sized the padded crop with no
limit; both are held to what the frame can use (blur at most 100, as the
options command allows). A Roto Brush span from the project made one render
build a key for every frame of it, rescanning all strokes per frame; the
chain reaches at most 200,000 frames either side of the base and strokes
are grouped by frame once. The camera solver copied the whole
reconstruction for every unsolved frame, even frames seeing too few points
to resect; those are skipped first.
* address review: in_span uses the MAX_CHAIN-bounded span, forward key range cannot overflow