Add ability to set texture and gradient backgrounds

[griffin28]

Add ability for all renderers to use texture and gradient backgrounds. Can overload the backgroundColor parameter with different types to support the different backgrounds.

Name	Type
backgroundColor	Color/Sampler/ARRAY1D of Color

• Color will be the current FLOAT32_VEC4 background color implementation
• Sampler => texture background
• ARRAY1D of Color => will have the list of colors for the gradient background

[jeffamstutz]

Implemented in be48467.