IECoreRenderMan : Support overscan

[danieldresser-ie]

Pretty straightforward, with just one weird floating point rounding thing, which has been documented and probably tested enough that there aren't any problems ( at least not with images smaller than 16 million pixels across ).

( Recreated with branch in the main repo, so the renderer tests run ).

[johnhaddon]

Thanks Daniel! I suspect this has exposed a bug in our new display driver for XPU, since overscan isn't coming out right in XPU interactive renders (RIS top, XPU bottom) :

Would be good to get that sorted before merging (seems like it might be related to the crop window code you mentioned you'd been looking at anyway).

[danieldresser-ie]

I've added an additional commit that overhauls the XPU display driver, addressing both your observation here and #6727.

I've tested it a fair bit interactively, and am pretty confident that it's working now, but I haven't yet added tests, and I also haven't yet addressed the non-XPU part of #6727 ( Not sure how important it is to deal correctly with changing the dataWindow interactively for non-XPU renders ).