[FEATURE] Add support for 1-, 2- or 3-channel textures.

[travisvroman]

Currently, the engine only supports 4-channel textures. This is not always needed or wanted. Add support for the following texture configurations:

• 1-channel (R8)
• 2-channel (R8B8)
• 3-channel (R8B8G8)

While at it, change default textures (default, default diffuse, default specular and default normal) to use 3-channel textures.