Fix premultiplied alpha issue with color correction
When correcting a color that was with premultiplied alpha, the alpha
value was not multiplied back again as a final step. This was breaking
color correction when the blend function was GL_ONE,
GL_ONE_MINUS_SRC_ALPHA. The blend function was changed for normal
windows (a workaround), but not for effect frames, i.e. the effect
frames were broken with color correction enabled.
Removes the blend function workaround.
Removes a useless setupForOutput.