Patch for: T31531

In old and new compositors, RGB is converted to BW as follows:

Old compositor:

*out= 0.333333f*(in[0]+in[1]+in[2]);

or

*out= in[0]*0.35f + in[1]*0.45f + in[2]*0.2f;

New compositor:

outputValue[0] = ((inputColor[0] + inputColor[1] + inputColor[2])/3.0f)*inputColor[3];

or

outputValue[0] = (inputColor[0]*0.35f + inputColor[1]*0.45f + inputColor[2]*0.2f)*inputColor[3];

So, obviously the old and new compositors produce the different results when alpha is less than 1.0.
This patch modifies these calculations not to multiply the brightness by the alpha component.