Add "sample locations" feature to GrProcessor

src/gpu/glsl/GrGLSLFragmentShaderBuilder.h

Index: src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
diff --git a/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h b/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
index b9816af71a2a014ea8eb1388443078993c8fa4a5..a1e994bfce4ec4cb8bf53675999bf7ba3849980c 100644
--- a/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
+++ b/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
@@ -67,6 +67,15 @@ public:
     GrGLSLFPFragmentBuilder() : GrGLSLFragmentBuilder(nullptr) {}
 
     /**
+     * Appends the offset, in device space, from the center of the pixel to a specified sample.
+     *
+     * @param sampleIdx    GLSL expression of the sample index.
+     *
+     * Requires a pipeline with sample locations enabled.
+     */
+    virtual void appendSampleOffset(const char* sampleIdx) = 0;
+
+    /**
      * Subtracts sample coverage from the fragment. Any sample whose corresponding bit is not found
      * in the mask will not be written out to the framebuffer.
      *
@@ -152,6 +161,7 @@ public:
     const char* fragmentPosition() override;
 
     // GrGLSLFPFragmentBuilder interface.
+    void appendSampleOffset(const char* sampleIdx) override;
     void maskSampleCoverage(const char* mask, bool invert = false) override;
     void overrideSampleCoverage(const char* mask) override;
     const SkString& getMangleString() const override { return fMangleString; }