Modify MBT tutorial to show how to introduce a mask

[fspindle]

In vpMbGenericTracker there is the function setMask(const vpImage<bool> &mask) that is useful to exclude a part from the image from tracking. This image could be updated at each iteration of the tracker.

In the existing tutorial in the Advanced section we should introduce a new subsection How to introduce a tracking mask

An image with a cluttered environment should be given as illustration. Why not a cube on a table with a lot of things on the table in the first plane that could be excluded using the mask.