ccpr: Use Manhattan distance for curve AA

Manhattan distance has the nice property that every pixel crossed
anywhere by the curve gets a fractional coverage value, whereas pixels
not touched by the curve will have zero coverage. This arguably looks
better on its own merits, but it also helps the curves blend more
seamlessly with their adjoining edges, which are already using Manhattan
distance.

Bug: skia:
Change-Id: I31cf28171d0b74512c74dca1088e50f0f442b924
Reviewed-on: https://skia-review.googlesource.com/115781
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>

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