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

src/gpu/GrGeometryProcessor.cpp

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "GrGeometryProcessor.h"
+#include "gl/GrGLGeometryProcessor.h"
+#include "gl/builders/GrGLProgramBuilder.h"
+#include "gl/builders/GrGLVertexShaderBuilder.h"
+
+const GrGeometryProcessor* GrGeometryProcessor::CreatePassthroughGP(size_t vertexStride) {
+    class NullGP : public GrGeometryProcessor {
+    public:
+        NullGP(size_t vertexStride) {
+            this->initClassID<NullGP>();
+            fPosAttr = &this->addVertexAttrib(Attribute("inPosition", kVec2f_GrVertexAttribType));
+            // Must call this after setting fPosAttr as that function adds 2 floats to the vertex
+            // stride.
+            SkASSERT(vertexStride >= sizeof(float) * 2);
+            fVertexStride = vertexStride;
+        }
+
+        const char* name() const override { return "Null"; }
+
+        void getGLProcessorKey(const GrGLSLCaps& caps, GrProcessorKeyBuilder* b) const override {}
+
+        GrGLPrimitiveProcessor* createGLInstance(const GrGLSLCaps& caps) const override {
+            class GLGP : public GrGLGeometryProcessor {
+            private:
+                void onEmitCode(EmitArgs& emitArgs, GrGPArgs* gpArgs) override {
+                    SkASSERT(!emitArgs.fTransformsIn.count());
+                    const NullGP& gp = emitArgs.fGP.cast<NullGP>();
+                    GrGLGPBuilder* pb = emitArgs.fPB;
+                    GrGLVertexBuilder* vsBuilder = pb->getVertexShaderBuilder();
+                    vsBuilder->emitAttributes(gp);
+                    this->setupPosition(pb, gpArgs, gp.fPosAttr->fName);
+                }
+                void setData(const GrGLProgramDataManager&, const GrPrimitiveProcessor&) override {}
+            };
+            return new GLGP();
+        }
+    private:
+        const Attribute* fPosAttr;
+    };
+    return new NullGP(vertexStride);
+}