Fix stroked roundrects on Nexus 10

Fix stroked roundrects on Nexus 10.

Due to floating point precision errors, using the formula b^2x^2 + a^2y^2  - a^2b^2 
doesn't work on the Nexus 10 GPU. Changed to use x^2/a^2 + y^2/b^2  - 1. 

Addresses issue 1290.

Committed: http://code.google.com/p/skia/source/detail?r=9110

[bsalomon, 2013-05-10 19:53:31]

On 2013/05/10 19:41:35, JimVV wrote:

lgtm

[commit-bot: I haz the power, 2013-05-13 14:26:51]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/jvanverth@google.com/15039016/1

[commit-bot: I haz the power, 2013-05-13 14:41:06]

Change committed as 9110