batch merge new edges

Find where newly introduced edges go in the edge list once,
then stitch all of them into the edge list.

R=reed@google.com
BUG=573166
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1647223002

Committed: https://skia.googlesource.com/skia/+/ae658e15477df86d1a864feb48d0274af2784f40

Review URL: https://codereview.chromium.org/1647223002
1 file changed