combine vertical overlapping edges

Paths outside clips, and sometimes paths inside clips, devolve
to multiple adjacent or overlapping vertical edges. Combine
these edges when possible to reduce the overall edge count.

R=reed@google.com
BUG=573166
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1654433002

Review URL: https://codereview.chromium.org/1654433002
2 files changed