Make GrGLShaderBuilder responsible for enabling GLSL extensions

src/gpu/gl/GrGLShaderBuilder.h

Index: src/gpu/gl/GrGLShaderBuilder.h
===================================================================
--- src/gpu/gl/GrGLShaderBuilder.h	(revision 8458)
+++ src/gpu/gl/GrGLShaderBuilder.h	(working copy)
@@ -107,6 +107,22 @@
     GrGLShaderBuilder(const GrGLContextInfo&, GrGLUniformManager&, const GrGLProgramDesc&);
 
     /**
+     * Use of these features may require a GLSL extension to be enabled. Shaders may not compile
+     * if code is added that uses one of these features without calling enableFeature()
+     */
+    enum GLSLFeature {
+        kStandardDerivatives_GLSLFeature = 0,
+
+        kLastGLSLFeature = kStandardDerivatives_GLSLFeature
+    };
+
+    /**
+     * If the feature is supported then true is returned and any necessary #extension declarations
+     * are added to the shaders. If the feature is not supported then false will be returned.
+     */
+    bool enableFeature(GLSLFeature);
+
+    /**
      * Called by GrGLEffects to add code to one of the shaders.
      */
     void vsCodeAppendf(const char format[], ...) SK_PRINTF_LIKE(2, 3) {
@@ -313,7 +329,6 @@
     // TODO: Everything below here private.
 public:
 
-    SkString    fHeader; // VS+FS, GLSL version, etc
     VarArray    fVSAttrs;
     VarArray    fVSOutputs;
     VarArray    fGSInputs;
@@ -327,6 +342,18 @@
         kNonStageIdx = -1,
     };
 
+    /**
+     * Features that should only be enabled by GrGLShaderBuilder itself.
+     */
+    enum GLSLPrivateFeature {
+        kFragCoordConventions_GLSLPrivateFeature = kLastGLSLFeature + 1
+    };
+    bool enablePrivateFeature(GLSLPrivateFeature);
+
+    // If we ever have VS/GS features we can expand this to take a bitmask of ShaderType and track
+    // the enables separately for each shader.
+    void addFSFeature(uint32_t featureBit, const char* extensionName);
+
     // Interpretation of DstReadKey when generating code
     enum {
         kNoDstRead_DstReadKey         = 0,
@@ -338,6 +365,7 @@
     const GrGLContextInfo&              fCtxInfo;
     GrGLUniformManager&                 fUniformManager;
     int                                 fCurrentStageIdx;
+    uint32_t                            fFSFeaturesAddedMask;
     SkString                            fFSFunctions;
     SkString                            fFSHeader;