Add implementation of path ops

Add implementation of path ops

This CL depends on
https://codereview.chromium.org/12880016/
"Add intersections for path ops"

Given a path, iterate through its contour, and
construct an array of segments containing its curves.

Intersect each curve with every other curve, and for
cubics, with itself.

Given the set of intersections, find one with the 
smallest y and sort the curves eminating from the
intersection. Assign each curve a winding value.

Operate on the curves, keeping and discarding them
according to the current operation and the sum of
the winding values.

Assemble the kept curves into an output path.
Committed: https://code.google.com/p/skia/source/detail?r=8553

[caryclark, 2013-04-08 11:50:57]

Committed manually as r8553 (presubmit successful).