Fix thin rect stroking.

When both edges of a thin (sub-unit) stroke fall within the same pixel, we compute the wrong coverage and also blit the same line twice.

To avoid these issues, tweak the inner/outer hulls to ensure one of the edges is always pixel-aligned.

BUG=skia:3278
R=reed@google.com

Review URL: https://codereview.chromium.org/819253003
2 files changed