Add passthrough GP for cases where no color or coverage should be emitted by GP

src/gpu/GrGeometryProcessor.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGeometryProcessor_DEFINED
 #define GrGeometryProcessor_DEFINED
 
 #include "GrPrimitiveProcessor.h"
 
 /**
  * A GrGeometryProcessor is a flexible method for rendering a primitive.  The GrGeometryProcessor
  * has complete control over vertex attributes and uniforms(aside from the render target) but it
  * must obey the same contract as any GrPrimitiveProcessor, specifically it must emit a color and
  * coverage into the fragment shader.  Where this color and coverage come from is completely the
  * responsibility of the GrGeometryProcessor.
  */
 class GrGeometryProcessor : public GrPrimitiveProcessor {
 public:
+    /**
+     * Creates a simple geometry processor that reads device-space positions as the first vertex
+     *  attribute (vec1, offset=0) and passes it through. It does no color or coverage calculation

[egdaniel, 2015/10/14 21:22:13]

extra space at start of comment?

+     *  and assumes no coord transforms are required by any FPs.
+     */
+    static const GrGeometryProcessor* CreatePassthroughGP(size_t vertexStride);
+
     GrGeometryProcessor()
         : INHERITED(false)
         , fWillUseGeoShader(false)
         , fLocalCoordsType(kUnused_LocalCoordsType) {}
 
     bool willUseGeoShader() const override { return fWillUseGeoShader; }
 
     bool hasTransformedLocalCoords() const override {
         return kHasTransformed_LocalCoordsType == fLocalCoordsType;
     }
@@ skipping 45 matching lines @@
     }
 
 private:
     bool fWillUseGeoShader;
     LocalCoordsType fLocalCoordsType;
 
     typedef GrPrimitiveProcessor INHERITED;
 };
 
 #endif