Make transparent screenshader screens possible again

- Allow alpha in clear color set. 
In the last few commits something changed, before the clearcolor was ignored for screens/scenes with PostFilters, afterward they started being applied. But! This is a problem because the current setClearColor doesn't allow alpha, so a see-through Screen with a post filter was not possible anymore. Add an alpha arg.

- Set clear color before-- not after-- clear
Why on earth was it doing it this way before?