New histogram equalization filter

New filter that performs approximate histogram equalization of a single channel, all three channels separately (leading to changes in color balance), or just the luma channel (attempting to keep hue and saturation the same). At present the filter performs slower than optimal and could benefit from a move to OpenGL ES 3.0 - the histogram has to be processed using the CPU as OpenGL ES 2.0 under iOS does not provide the necessary precision. 

See http://en.wikipedia.org/wiki/Histogram_equalization for more information.