fix quadclipper in the case that the chop function fails

fix quadclipper in the case that the chop function fails

If the chopper fails, then we've hit some numerical edge-case, which indicates
that the quad is just barely crossing the edge, so to handle that, we just
clamp the Y values to the edge. This distorts the quad, but only in the case
when 99% of the quad will not be affected.

git-svn-id: http://skia.googlecode.com/svn/trunk@404 2bbb7eff-a529-9590-31e7-b0007b416f81