new scanconversion technique

new scanconversion technique

This technique geometrically clips all segments against the clip bounds,
ensuring that we never send a value to the edgelist that might overflow in
fixedpoint.

Current disabled in SkScan_Path.cpp by a #define. There are a few minor pixel
differences between this and the old technique, as found by the gm tool, so
at the moment this new code is off by default.

git-svn-id: http://skia.googlecode.com/svn/trunk@432 2bbb7eff-a529-9590-31e7-b0007b416f81