Add clip effect for circles

Add clip effect for circles

BUG=skia:2181
Committed: http://code.google.com/p/skia/source/detail?r=13974

[bsalomon, 2014-03-27 17:05:10]

I think the try bot failures are all related to the nvpr text context and not
this CL.

[jvanverth1, 2014-03-27 20:05:49]

LGTM.  It seems like you should be able to infer the center of the circle
somehow, from the geometry, but I'm not sure if that would help the speed.

[bsalomon, 2014-03-27 20:09:33]

On 2014/03/27 20:05:49, JimVV wrote:
> LGTM.  It seems like you should be able to infer the center of the circle
> somehow, from the geometry, but I'm not sure if that would help the speed.

The geometry is arbitrary since this is used for clipping.

[bsalomon, 2014-03-28 14:37:10]

The CQ bit was checked by bsalomon@google.com

[commit-bot: I haz the power, 2014-03-28 14:37:16]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/bsalomon@google.com/214153002/40001

[commit-bot: I haz the power, 2014-03-28 15:58:51]

Change committed as 13974