No chop at y extrema for cubics

The new Delta AA scan converter does not need the edge to be updated
with monotonic Y so chopping at y extrema is not necessary. Removing
such chopping brings ~10% performance increase to chalkboard.svg which
has tons of small cubics (the same is true for many svgs I saw).

We didn't remove the chopping for quads because that does not bring
a significant speedup. Moreover, dropping those y extremas would make
our strokecircle animation look a little more wobbly (because we would
have fewer divisions for the quads at the top and bottom of the circle).

Bug: skia:
Change-Id: I3984d2619f9f77269ed24e8cbfa9f1429ebca4a8
Reviewed-on: https://skia-review.googlesource.com/31940
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>
7 files changed