updates for usd-0.19.07

[pmolodo]

Description (this won't be part of the changelog)

Fixes to build against usd-0.19.07

(Note that this PR branches off of the 0.19.05 fixes - https://github.com/AnimalLogic/AL_USDMaya/pull/150)

Changelog

Changed

• Builds against / requires usd-0.19.07

Checklist (Please do not remove this line)

• [X] Make sure the Title and Description of the PR make sense and provide sufficient context for your work
• [X] Do any added files have the correct AL Apache Licence Header?
• [X] Are there Doxygen comments in the headers?
• [X] Are any new features, behaviour changes documented in the .md format documentation?
• [X] Have you added, updated tests to cover new features and behaviour changes?
• [X] Have you filled out at least one changelog entry?
• [X] Is the branch's history clean? (only relevant commits)

[pmolodo]

The cull style is still being set - it's now just handled by the combo of:

HdxRenderTaskParams hdParams = _MakeHydraUsdImagingGLRenderParams(params);
_taskController->SetRenderParams(hdParams);

[Wouterdek]

Maybe docs/build.md could also be updated as this project doesn't work with 18.11 anymore but it's still listed as a requirement on the build instructions?