Fix vertex count estimate in GrTessellator.

When we start a new MonotonePoly due to a handedness change, we don't need to
increase the vertex count, since that edge (and vertex) has already been
accounted for in the previous MonotonePoly.

This was not a correctness issue, but was causing us to allocate
extra vertices which would go unused.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2146063002

Review-Url: https://codereview.chromium.org/2146063002
1 file changed