Port feMorphology, feDisplacementMap, feGaussianBlur, feTurbulence; fix dashed stroke offset on circles and some filter edge cases
- The displacement map tests suffer from the image-renders-PNGs-darker issue and will need to be regenerated when that's fixed.
- Fixed stroke offset on circles (they were being drawn the wrong way around).
- Fixed empty and non-existing filters returning the source graphic when they should return nothing.
- Fixed filters having effect on masks (please check this, I'm not sure it's the best place to handle that logic: this simply states that the
filterproperty does not apply to