OCL: ocl::cvtColor

- fixed ocl::cvtColor for CV_YUV2BGRA and CV_YUV2RGBA
- added YCrCb to RGB, BGR, RGBA, BGRA color space conversions.
- color.cpp refactoring: created generic interface for toRGB and fromRGB callers
- added RGB, RGBA, BGR, BGRA <-> XYZ conversion
- added RGB[A] <-> BGR[A] modes

etcetera (see below)