--PBR Primitive support and gl context bugfix

[jturner65]

Motivation and Context

This PR introduces PBR material support and generation for primitives and generated meshes such as trajectories, which were formerly restricted to just phong materials, which looked out of place in scenes that were predominantly rendered using PBR with image-based lighting. This also adds vertex color support for PBR materials. Lastly, this also fixes a bug that was present if trying to build a PBR containing scene without a context (which was being done in a few of the tests).

This functionality was originally going to be part of the PBRTesting PR but I chose to go a different direction with that test due to the difficulty in assigning custom materials to primitive assets on creation.

How Has This Been Tested

C++ and python tests pass locally.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

[jturner65]

PBR primitives with default IBL environment map.

PBR primitives in the same scene with alternate IBL environment map

Same scene with yet another different environment map.

[jturner65]

Vertex color demo via a gradient on a generated metallic trajectory