handle halfway case in scan converter

handle halfway case in scan converter

Scan edges that start at exactly -0.5 aren't trimmed by
clipping or by rounding, triggering a debug assert.

One way to fix this is to round the top and left
down instead of up.

Also, move the path initialization of gm/composeshader.cpp
to make debugging other path problems easier.

R=reed@google.com
BUG=skia:2715
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1544873002

Review URL: https://codereview.chromium.org/1544873002