New features:
- replace OpenGL/glfw by Metal/OpenGL backends
- clear Engine instead of delete when plugin changes
- add ambient and dome lights support + update layout
- add color correction (OpenGL only)
- emissive UsdPreviewSurface for Grid
- create Active Scene Index, Scene Index View, Scene Index Attribute
- update the doc and resources
Fixes:
- fix crash when load usd file with non empty stage
- fix crash when UsdStage cleared
- add X11 dependency
- add name to scene indices
Cleanup:
- cleanup UsdStage from Model
- remove unused imGuizmo DrawGrid
- remove various useless pxr:: namespaces
- cleanup Engine and remove unused methods