Stop discarding in frag shader when coverage is zero and stencil writes are enabled

This fixes the corruption in the NVPR images here:
http://108.170.217.252:10117/builders/Test-Win8-ShuttleA-GTX660-x86-Debug/builds/251/steps/CompareGMs/logs/stdio

caused by:

https://skia.googlesource.com/skia.git/+/65ee5f424cb4dabd453268902c00086605d77c1d

The stencil path step writes inside and outside the clip when the clip is implemented by
a coverage effect. The path cover step then doesn't write outside of the clip because the FS
discards. This leaves stencil values outside of the clip non-zero which messed up subsequent
path or clip draws to those samples.

R=robertphillips@google.com

Author: bsalomon@google.com

Review URL: https://codereview.chromium.org/154623002

git-svn-id: http://skia.googlecode.com/svn/trunk@13320 2bbb7eff-a529-9590-31e7-b0007b416f81
4 files changed