Minor PixelRoutine cleanup

Minor PixelRoutine cleanup

- Removed redundant parameters from fogBlend()
- Moved computation registers and color output
  registers from PixelRoutine to PixelProgram.
- Made many PixelRoutine member functions
  private rather than protected when possible.

Change-Id: I748333626d993c0d46b369991d74bc3a22c972c2
Reviewed-on: https://swiftshader-review.googlesource.com/3850
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>