Disable dual source blending support when GLSL version is too old

src/gpu/gl/GrGLSL.h

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGLSL_DEFINED
 #define GrGLSL_DEFINED
 
@@ skipping 32 matching lines @@
      * ES GLSL 3.10 only TODO Make GLSLCap objects to make this more granular
      */
     k310es_GrGLSLGeneration,
 };
 
 /**
  * Gets the most recent GLSL Generation compatible with the OpenGL context.
  */
 bool GrGetGLSLGeneration(const GrGLInterface* gl, GrGLSLGeneration* generation);
 
+bool GrGLSLSupportsNamedFragmentShaderOutputs(GrGLSLGeneration);
+
 /**
  * Returns a string to include at the beginning of a shader to declare the GLSL
  * version.
  */
 const char* GrGetGLSLVersionDecl(const GrGLContextInfo&);
 
 /**
  * Adds a line of GLSL code to declare the default precision for float types.
  */
 void GrGLSLAppendDefaultFloatPrecisionDeclaration(GrSLPrecision, GrGLStandard, SkString* out);
@@ skipping 273 matching lines @@
 
 /**
  * Does an inplace mul, *=, of vec4VarName by mulFactor.
  * A semicolon is added after the assignment.
  */
 void GrGLSLMulVarBy4f(SkString* outAppend, const char* vec4VarName, const GrGLSLExpr4& mulFactor);
 
 #include "GrGLSL_impl.h"
 
 #endif