[Bugfix] Specify default stage shader type as flat;

[jturner65]

Motivation and Context

Small bugfix that specifies a stage's default shader type to be flat instead of unknown. Since stages often come with lighting baked into the textures, we do not want to apply phong or pbr shading on top of this. This, of course, can be easily overridden, either at the stage config level, or for individual instancing scenes.

REVERTED BY 1311

How Has This Been Tested

All 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.