Materials for rendering abstract class

Add a new class for materials, which is attached to render delegates to enable the user to define material details for each object.
Initial abstract implementation:
- Textures (all of these are technically optional):
  - Diffuse
  - Normal
  - Specular (classical)
  - Metalness (PBR)
  - Roughness (PBR)
  - AO (optional)
  - Cavity (optional)
  - Cubemap (optional)
- State
  - Diffuse color (default is white)
  - Specular color (default is white)
  - Metalness (PBR, default to 0.0)
  - Roughness (PBR, default to 1.0)
  - Casts shadows
  - Receives shadows
  - Wireframe
  - Line width
- Shaders
  - Custom vertex shader (optional)
  - Custom geometry shader (optional)
  - Custom fragment shader (optional)