Option to render viewmodel separately and set clear color

[cademtz]

For every stream, SparklyFX will manually clear the frame buffer with a solid color and invoke rendering with flags to render world + viewmodel.

Instead, this should be configurable in a stream.
By setting the clear color green and rendering only the viewmodel, the green-screening process would be easier.

The frame buffer can also hold alpha which will be more reliable than green-screening.