Use stencil ops that don't issue writes whenever possible

src/gpu/GrStencilAndCoverTextContext.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrStencilAndCoverTextContext.h"
 #include "GrAtlasTextContext.h"
 #include "GrDrawContext.h"
@@ skipping 417 matching lines @@
                                                                  fLocalMatrix));
 
         // We should only be flushing about once every run.  However, if this impacts performance
         // we could move the creation of the GrPipelineBuilder earlier.
         GrPipelineBuilder pipelineBuilder(fPaint, fRenderTarget, fClip);
         SkASSERT(fRenderTarget->isStencilBufferMultisampled() || !fPaint.isAntiAlias());
         pipelineBuilder.setState(GrPipelineBuilder::kHWAntialias_Flag, fPaint.isAntiAlias());
 
         GR_STATIC_CONST_SAME_STENCIL(kStencilPass,
                                      kZero_StencilOp,
-                                     kZero_StencilOp,
+                                     kKeep_StencilOp,
                                      kNotEqual_StencilFunc,
                                      0xffff,
                                      0x0000,
                                      0xffff);
 
         *pipelineBuilder.stencil() = kStencilPass;
 
         SkASSERT(0 == fQueuedGlyphCount);
         SkASSERT(kGlyphBufferSize == fFallbackGlyphsIdx);
 
@@ skipping 38 matching lines @@
 
     fGlyphs->unref();
     fGlyphs = NULL;
 
     SkGlyphCache::AttachCache(fGlyphCache);
     fGlyphCache = NULL;
 
     fViewMatrix = fContextInitialMatrix;
 }