Distinguish the 3 different methods for acquiring pixel color values (executePixel, executePixelSampled, executePixelFiltered).
This makes it easier to keep track of the different sampling methods (and works nicer with IDEs that do code parsing).
Differential Revision: http://developer.blender.org/D7
calculate the distance in 3d between R,G and B, the YCC only takes Cb and Cr
into consideration.
This commit makes COM_DistanceMatteOperation inheritable and expose the calculate distance function
to be re-implemented for the YCC node operation.
Thanks Troy Sobotka for the report over email.
Patch incorporates review suggestions by Jeroen Bakker.
