GrTessellator (AA): improve AA quality for near-overlapping paths.

GrTessellator (AA): improve AA quality for near-overlapping paths.

When path features are very close together, the outer contours can
overlap. This causes the connector edges (the ones joining the inner and
outer vertices) to intersect other edges. Lerping the alpha along the
connector edge gives us a good approximation of the coverage at that
point.

BUG=skia:

Change-Id: I56bcc570fc185344c5f84d11ef995d3940a08793
Reviewed-on: https://skia-review.googlesource.com/7701
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Stephan White <senorblanco@chromium.org>