A class to collect boxes which intersect a given clip region. If available, aggregate bounds information will be used. Block and inline content are added to separate lists as they are painted in separate render phases. This class flattens the nested structure of boxes in a depth first fashion. Range lists are created for overflow:hidden boxes to specify the first block where a clip comes into affect and the end block where the clip is undone. For example, if a hidden box is first in a layer and contains 10 child boxes the range list will contain an entry for 0, 11, specifying the start and end of the clip.