Arnold IPR Crop Region improvement

[andrewkaufman]

Summary

When adjusting the crop region box in an Arnold IPR, it currently appears to start a new render by making a new entry in the catalogue. I would prefer if the crop tool instead updates the render region for the current render / catalogue entry.

Implementation notes

@johnhaddon says

Arnold is always opening new drivers for each new progressive pass, and we have logic to ignore that and reuse the ones we have. But we currently do have to make a new one when the crop changes because the data window is baked into our drivers. Should be fixable.

[johnhaddon]

Relevant code is here : https://github.com/ImageEngine/cortex/blob/master/contrib/IECoreArnold/src/IECoreArnold/outputDriver/OutputDriver.cpp#L210-L220. The problem is that the display and data window are fixed at the point we construct the IECore::DisplayDriver. We'll need some new API to change them after the fact - probably will have to be for Cortex 10.3. Might be a good idea if we allowed changing of the display window at the same time.