Support exclusion zones

[iso-ivanv]

This is a really great tool for checking images. However, sometimes there are dynamic areas in the actual image which is subject to verification. So a static expected image fixture won't do the job. A simple solution would be : before comparing the images draw a black rectangle on specified position on both expected and actual images so any difference would be eliminated. Another more elegant solution might be : mark the exclusion zones in the expected image in a certain way and the tool would simply ignore them . They can be marked with either maximum opacity alpha:255 or specific color which can serve as ignore color. Does this sound like a useful feature and what option seems better? Regards