Rework Overstroke GM to show differences between expected and actual

Draw the perpendiculars of the curve with the stroke width. See Cary's
SampleQuadStroker.cpp for what we are attempting to mimic.

BUG=589769,skia:5405,skia:5406
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2218773003

Review-Url: https://codereview.chromium.org/2218773003
1 file changed