Fix double blend in GrAAConvexPathRenderer

Updates GrAAConvexPathRenderer to not draw the interior fan of a
convex path when it does not exist (i.e when segment count <= 2).

TODO: We should also detect and combine colinear segments in order to
make sure we catch every case.

BUG=skia:

Review URL: https://codereview.chromium.org/1094293002
1 file changed