1. feTile: a 40x40 tile of a blue box, repeated over 300x120
2. feGaussianBlur + feOffset (a drop shadow) and feColorMatrix (greyscale)
3. feTile, the overflow case: a 1x1 tile over a 4000x4000 filter region on a 300x100 box
4. the end: every box above painted, no fault