Incorporate glStencilThenCover* nvpr methods

Incorporate glStencilThenCover* nvpr methods

Adds the glStencilThenCover* nvpr methods to GrGLInterface and starts
using them. When drawing multible paths, this will make it so we only
have to send the index/transform data once. It will also allow the
driver to save time internally.

The glStencilThenCover* methods are a newer addition, so they aren't
available on every driver. In the event that they are not present, we
emulate them using the existing glStencil*/glCover* methods.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/c8f520466701a14cf2fb86d3f2d3c70983cb4518

[bsalomon, 2014-07-29 20:48:38]

lgtm

[Chris Dalton, 2014-07-29 21:40:57]

The CQ bit was checked by cdalton@nvidia.com

[commit-bot: I haz the power, 2014-07-29 21:41:35]

CQ is trying da patch. Follow status at
 https://skia-tree-status.appspot.com/cq/cdalton@nvidia.com/423173004/1

[commit-bot: I haz the power, 2014-07-29 21:52:12]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  Build-Win-VS2013-x86-Debug-Trybot on tryserver.skia
(http://108.170.220.76:10117/builders/Build-Win-VS2013-x86-Debug-Trybot/builds...)

[commit-bot: I haz the power, 2014-07-29 21:56:28]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-07-29 21:56:29]

Try jobs failed on following builders:
  Build-Win-VS2013-x86-Debug-Trybot on tryserver.skia
(http://108.170.220.76:10117/builders/Build-Win-VS2013-x86-Debug-Trybot/builds...)

[Chris Dalton, 2014-07-29 22:11:50]

The CQ bit was checked by cdalton@nvidia.com

[commit-bot: I haz the power, 2014-07-29 22:12:42]

CQ is trying da patch. Follow status at
 https://skia-tree-status.appspot.com/cq/cdalton@nvidia.com/423173004/20001

[commit-bot: I haz the power, 2014-07-29 22:25:59]

Change committed as c8f520466701a14cf2fb86d3f2d3c70983cb4518