Lighting and gaussian blur performance improvements

• Lighting code changes:

  • Merged diffuse and specular filter code, they were the same for the most part and it'd be inconvenient to apply changes to the code structure to both places at once.
  • Unrolled the lighting normal computation, now it looks more like in WebKit.
  • Optimized the common case of (0, 0, 1) normal (there's a comment in WebKit saying it's a common case, and also I checked the phone and it's indeed (0, 0, 1) for the most part).
  • Moved some coordinate transformations which didn't need to be inside the loops out of the loops.
  • Parallelized the computation with rayon.
  • This dropped the phone from 7.6 to about 6 seconds IIRC.

• Blur code changes:

  • Parallelized with some hacks and rayon. More details in the comments.
  • This dropped the phone to about 4 seconds.
  • This also dropped the box_blur benchmarks by about 50%.
  • And also the -p 1 -r 100 rsvg-bench on bugs/634324-blur-negative-transform.svg is now about 1.3 seconds, it was 2.8 seconds after !99 (merged).