Fixes to build against usd-0.19.5

[pmolodo]

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

• usd-0.19.05 now needs GlfContextCaps::InitInstance() called anywhere GlfGlewInit() is

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]

Was seeing an error message printed to console when loading in non-gui maya - fixed by a check against the GL context