Allow histograms (cvCalcHist()) to work with COI and/or iplImageHeader (Feature #2929)

Currently, computing the R,G,B histograms for a color image requires the use of cvSplit(), and full copy for each channel.
If it is still compatible with the inner code of cvCalcHist(), I think it would be interesting to make it compatible with COI and IplImageHeader. The following lazy code could then be applied, requiring no data copy and better performance :

IplImage* srcImage3Channels = ...;
IplImage* channel1 = cvCreateImageHeader(...);
cvSetData(channel1 , srcImage3Channels->data,...);
cvSetImageCOI(channel1 , 1);
IplImage* channel2 = cvCreateImageHeader(...);
cvSetData(channel2 , srcImage3Channels->data,...);
cvSetImageCOI(channel2 , 2);
IplImage* channel3 = cvCreateImageHeader(...);
cvSetData(channel3 , srcImage3Channels->data,...);
cvSetImageCOI(channel3 , 3);
cvCalcHist(channel1...);
cvCalcHist(channel2...);
cvCalcHist(channel3...);