Map a drawn line to the other camera only once both ends are placed

[mattdawkins]

• the first end of a head/tail line being drawn was mapped to the other stereo camera on its own (Support editable curved centerlines and stereo keypoint mapping #1918); the mid-draw transfer interrupted the line draw and flashed the stereo dialog. It now waits for the whole-line transfer. A lone point that is dragged later still maps.
• the "Computing stereo correspondence..." dialog only appears once a transfer has taken 400 ms, and never for a single point; errors still show
• a transfer skipped mid-flight no longer leaves the spinner dialog open
• spec covers the deferred mapping

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