[postprocessing] r_clearbuffer no longer works

[Rachael]

r_clearbuffer - which is an extremely useful level diagnostic tool - no longer works since the postprocessing additions to GZDoom.

It took multiple parameters, allowing it to draw black, white, flashing, or cycling through the current game's palette. It cleared the backdrop to make HOM errors on a level easier to detect.

[dpJudas]

Pull request fixing this: https://github.com/coelckers/gzdoom/pull/83

Note that mode 0 does not generate HOMs anymore in gzdoom. This is for performance reasons, where clearing buffers between frames is faster than not doing it on newer hardware.