simplify convexity-helper code

Change-Id: I7dd015a1cf76570bc2a6f00601800c46a3bd88a4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/239926
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
Auto-Submit: Mike Reed <reed@google.com>
1 file changed