add interp path

Add path methods to determine if a pair of paths
can be interpolated, and to interpolate them.

R=reed@google.com, robertphillips@google.com
BUG=skia:4549
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1703943003

Review URL: https://codereview.chromium.org/1703943003
5 files changed