fix large dashed paths

Paths with lots of points exceed the 32767 point index.
Make the index larger, add a test, and allow the
t value to use the extra bits.

R=reed@google.com,fmalita@chromium.org
BUG=skia:3501
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1534223002

Review URL: https://codereview.chromium.org/1534223002
3 files changed