enable early Z for SVG.
Additionally, this switches the B-quad patches for XCAA to be convex
hulls instead of bounding boxes, reducing fragment shader load.
This is a large speedup for the Ghostscript tiger demo: 5x or more.
Closes#33.
ECAA algorithm for multicolor SVG.
This is much faster than both Skia and the previous XCAA algorithm
while maintaining slightly higher quality than the latter.
There are a couple of known issues:
* Vertical inflection points of hairlines with very steep slopes can
become very light or even drop out occasionally. I suspect this is due
to floating point error.
* Rarely, single columns of pixels can disappear from a mesh. Cause TBD.
Besides these bugs, this technique can be cleaned up and probably made
faster, but it's a sizable improvement as is.
Patrick Walton