fix issue where proxy shape wasn't redrawn when filepath was changed

[pmolodo]

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

Currently, if you alter the filepath of ProxyShape that's already been drawn, then nothing will happen in the viewport - ie, it will keep displaying the stage.

I fixed this by making the gl engine constructed on demand (and destroying any old engine after loading a stage). I also made sure we didn't store refs to the engine, but always query it from the ProxyShape.

(Note: I confirmed that this is not related to the recent changes which were merged, including https://github.com/AnimalLogic/AL_USDMaya/pull/89).

Changelog

Fixed

• fix issue where proxy shape wasn't redrawn when filepath was changed

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?
• [ ] 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)