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:
R=markkilgard@gmail.com, bsalomon@google.com

Author: cdalton@nvidia.com

Review URL: https://codereview.chromium.org/423173004
diff --git a/src/gpu/gl/GrGLInterface.cpp b/src/gpu/gl/GrGLInterface.cpp
index 823b73e..e9ba32e 100644
--- a/src/gpu/gl/GrGLInterface.cpp
+++ b/src/gpu/gl/GrGLInterface.cpp
@@ -467,7 +467,11 @@
             NULL == fFunctions.fCoverFillPath ||
             NULL == fFunctions.fCoverStrokePath ||
             NULL == fFunctions.fCoverFillPathInstanced ||
-            NULL == fFunctions.fCoverStrokePathInstanced) {
+            NULL == fFunctions.fCoverStrokePathInstanced ||
+            NULL == fFunctions.fStencilThenCoverFillPath ||
+            NULL == fFunctions.fStencilThenCoverStrokePath ||
+            NULL == fFunctions.fStencilThenCoverFillPathInstanced ||
+            NULL == fFunctions.fStencilThenCoverStrokePathInstanced) {
             RETURN_FALSE_INTERFACE
         }
         // Currently ProgramPathFragmentInputGen is not used on