When comparing histograms that look like multi-channel images (e.g a 3D histogram, of 4x4x4 bins, might appear as a CV_32FC4 matrix), cv::compareHist would complain because it was expecting the matrix type() == CV_32F. Now we test matrix depth() == CV_32F instead.

This reverts commit fba72cb6, reversing
changes made to 02131ffb.