[NPHVJ] Update view ROI after view moves, not just on mouseup

[Ben Karel]

Currently, NPHVJ sends a new region of interest request on mouseup only. This should be generalized so that when the viewport is updated by any means, including keyboard or JS, the ROI may also be updated.

The advantage of mouseup is that it serves as a marker for the user "settling" on a particular location; we can be relatively sure that the viewport will stop moving for at least a few tenths of a second after a mouseup. To avoid doing unnecessary work rendering a ROI that will immediately be moved, a replaceable timeout should be set on each view move.

This could be combined with a strategy of requesting and rendering very small ROIs immediately (every few pixels, typically), with the larger/complete ROI requested after some cooldown period.

[Ben Karel (eschew)]

Initial fix in revision 363 of branch nphvj.