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.

R=bsalomon@google.com

Author: jvanverth@google.com

Review URL: https://chromiumcodereview.appspot.com/15039016

git-svn-id: http://skia.googlecode.com/svn/trunk@9110 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed