Add some backend flexibility for shader declarations in ProgramBuilding.

src/gpu/glsl/GrGLSLProgramBuilder.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGLSLProgramBuilder_DEFINED
 #define GrGLSLProgramBuilder_DEFINED
 
@@ skipping 51 matching lines @@
 
     // Generates a name for a variable. The generated string will be name prefixed by the prefix
     // char (unless the prefix is '\0'). It also will mangle the name to be stage-specific unless
     // explicitly asked not to.
     void nameVariable(SkString* out, char prefix, const char* name, bool mangle = true);
 
     virtual GrGLSLUniformHandler* uniformHandler() = 0;
     virtual const GrGLSLUniformHandler* uniformHandler() const = 0;
     virtual GrGLSLVaryingHandler* varyingHandler() = 0;
 
+    // Used for backend customization of the output color and secondary color variables from the
+    // fragment processor. Only used if the outputs are explicitly declared in the shaders
+    virtual void finalizeFragmentOutputColor(GrGLSLShaderVar& outputColor) {}
+    virtual void finalizeFragmentSecondaryColor(GrGLSLShaderVar& outputColor) {}
+
     // number of each input/output type in a single allocation block, used by many builders
     static const int kVarsPerBlock;
 
     GrGLSLVertexBuilder         fVS;
     GrGLSLGeometryBuilder       fGS;
     GrGLSLFragmentShaderBuilder fFS;
 
     int fStageIndex;
 
     const DrawArgs& fArgs;
@@ skipping 58 matching lines @@
     void verify(const GrFragmentProcessor&);
 
     virtual void emitSamplers(const GrProcessor& processor,
                               GrGLSLTextureSampler::TextureSamplerArray* outSamplers) = 0;
 
     GrGLSLPrimitiveProcessor::TransformsIn  fCoordTransforms;
     GrGLSLPrimitiveProcessor::TransformsOut fOutCoords;
 };
 
 #endif