Improve shader support and configuration

[aclegg3]

Shader support:

Habitat currently supports flat shading and vertex colored rendering of scenes and objects assuming that lighting is baked into object textures. Many scene and object datasets do not include lighting information in their color, texture, and/or material and are therefore not rendered realistically under this assumption.

Supporting shaders with illumination models will allow such assets to be rendered as intended and greatly increase overall asset compatibility and user experience.

Shader Configuration:

Habitat sim presently relies on automatic parsing of asset features (such as filetype and the presence of materials/textures) to select shaders for rendering scenes and objects. With improved shader support, automation becomes more likely to fail as the number of diverging use cases increases. In addition to shader type, users may prefer to manually configure shader parameters such as light position, intensity, color, relative coordinate frame, etc...

Supporting programmatically configurable shader setups would alleviate issues caused by incorrect shader configurations computed automatically. It would also provide greater support for use cases involving domain randomization, dynamic scene generation, and/or unknown/untested datasets.

Short list of desired features:

• Support for Magnum Phong illumination shader
• Programatic configuration of shader parameters
• Configurable assignment of scenes/objects to shader groups (to override automatic assignment)