It's a waste of GPU time to do two fullscreen passes to render final mix
previews. Use blend states to simulate the black background of
DrawBackdrop() for the following situations:

- Main preview window (Studio Mode off)
- Studio Mode: Program

This does not effect:

- Studio Mode: Preview (still uses DrawBackdrop)
- Fullscreen Projector (uses GPU clear to black)
- Windowed Projector (uses GPU clear to black)

intel GPA, SetStablePowerState, Intel HD Graphics 530, 1920x1080

Before:
DrawBackdrop: ~529 us
main texture: ~367 us (Cheaper than drawing a black quad?)

After:
[DrawBackdrop optimized away]
main texture: ~383 us