Color Conversion Matrices

Add the ability to apply color conversion matrices with an optional gamma to an image. This is already possible using pixelmath but this approach makes it more convenient than having to define 3 separate pm functions for r, g and b and includes an optional gamma parameter. This does represent an advanced, low-level approach to color manipulation but there are users who are doing this kind of low level manipulation. The provision of a ccm command means that presets can easily be saved as simple scripts. There is an accompanying doc MR.