[mtoh] Fix stage-repopulation.

[marsupial]

When rendering via mtoh, changes to UsdProxy.filepath aren't working.
(If not always, at least when VP_RENDER_DELEGATE=0)

[kxl-adsk]

Similar problem we were solving in VP2RenderDelegate - see condition https://github.com/Autodesk/maya-usd/blob/dev/lib/mayaUsd/render/vp2RenderDelegate/proxyRenderDelegate.cpp#L421 and implementation details for ProxyRenderDelegate::ProxyShapeData::IsUsdStageUpToDate() and ProxyRenderDelegate::ProxyShapeData::IsExcludePrimsUpToDate()

I will ask @williamkrick to review this change and point out opportunities for aligning the two render paths on the same mechanism to detect when stage on proxy shape changed.

[williamkrick]

Similar problem we were solving in VP2RenderDelegate - see condition https://github.com/Autodesk/maya-usd/blob/dev/lib/mayaUsd/render/vp2RenderDelegate/proxyRenderDelegate.cpp#L421 and implementation details for ProxyRenderDelegate::ProxyShapeData::IsUsdStageUpToDate() and ProxyRenderDelegate::ProxyShapeData::IsExcludePrimsUpToDate()

I will ask @williamkrick to review this change and point out opportunities for aligning the two render paths on the same mechanism to detect when stage on proxy shape changed.

My approach in Vp2RenderDelegate is to hold all the data we use from the ProxyShape on a class (ProxyShapeData) and do all the version tracking there. ProxyShapeData could be refactored into ProxyShapeBase or a similar common area and re-used by mtoh. ProxyShapeData can directly compare cached values to the current ProxyShapeBase data to detect changes in simple types, or compare versionIDs to detect changes in complex types.

Right now y'all have a callback registered TfNotice::Register(me, &HdMayaProxyAdapter::_OnStageSet); so so know exactly when the proxy is changing. I'm not sure where that system comes from, we don't use that in Vp2RenderDelegate.

[kxl-adsk]

It's true that the base proxy shape currently always has a valid stage. This doesn't have to be true a) in future, b) for plugin proxy shapes (example AL proxy shape). Please bring back this validation.

[kxl-adsk]

Changing the status of the review to "request changes"