Extend existing algorithms to work with color images #500
Description
The following algorithms implemented in vpDot, vpDot2, vpMbGenericTracker and may be others (vpMeLine, vpTemplateTracker...) cannot take a color image vpImage<vpRGBa> as input.
We should add the ability to pass a color image to these trackers, internally convert the color image in a gray level image vpImage<unsigned char> to apply the existing algorithm and display the tracking results over the color image.