Particularly the blur and blur-like (i.e. radius-based) filters will perform their coverage calculations when computing coverages, but it isn't clear that is the right answer. While they are 2D operations, their results should be as if they did their work on the 2D input and were then perspective transformed - which means that the distance of their radius-based effects will be bent by the perspective distortion.

We should double-check that they compute the right results and adjust the math accordingly.