video: Add public util function(s) for color transform matrix calculation
Some color transform matrix related functions in video-converter.c seems to be reusable by other color conversion elements.
I recently worked for color conversion elements cudacolorconvert and d3d11colorconvert and both elements uses (almost?) the same code of video-converter.c for calculating color conversion matrix. The glcolorconvert is also possible use case of that util function.