Problem this feature should fix

It's currently possible to approximate emissive results by boosting the albedo past 1.0.
Instead, we should add support for a proper emissive workflow.

Expected solution

Add support for:

• Emissive Map
• Emissive Color
• Emissive Strength

One issue that may arise is that when no texture is provided, Overload automatically fills this uniform with a pure white texture, which will cause the emissive to be fully white if no emissive map is provided. One way to mitigate that issue is to set the emissive parameters in a feature, so they are off by default. (Related to #575)