path ops : make it real

path ops : make it real

Add an option to SkCanvas to turn on path
ops when combining clips. 

Allow Op() to use one of the input paths
as an output path.

Fix a bug in Op() when the minuend is empty
and the subtrahend is not (for difference).

Change the build to allow core to depend on pathops.
Committed: https://code.google.com/p/skia/source/detail?r=8855

[reed1, 2013-04-25 13:23:56]

lgtm

[caryclark, 2013-04-25 13:34:47]

Committed patchset #3 manually as r8855 (presubmit successful).