Add "sample locations" feature to GrProcessor

src/gpu/glsl/GrGLSLFragmentShaderBuilder.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGLSLFragmentShaderBuilder_DEFINED
 #define GrGLSLFragmentShaderBuilder_DEFINED
 
@@ skipping 49 matching lines @@
 
 /*
  * This class is used by fragment processors to build their fragment code.
  */
 class GrGLSLFPFragmentBuilder : virtual public GrGLSLFragmentBuilder {
 public:
     /** Appease the compiler; the derived class initializes GrGLSLFragmentBuilder. */
     GrGLSLFPFragmentBuilder() : GrGLSLFragmentBuilder(nullptr) {}
 
     /**
+     * Appends the location of a specified sample. A sample location is understood to be its offset
+     * in device space from the center of the pixel.
+     *
+     * @param sampleIdx    GLSL expression of the sample index.
+     *
+     * Requires a pipeline with sample locations enabled.
+     */
+    virtual void appendSampleLocation(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.
      *
      * @param mask      int that contains the sample mask. Bit N corresponds to the Nth sample.
      * @param invert    perform a bit-wise NOT on the provided mask before applying it?
      *
      * Requires GLSL support for sample variables.
      */
     virtual void maskSampleCoverage(const char* mask, bool invert = false) = 0;
 
@@ skipping 65 matching lines @@
 
     GrGLSLFragmentShaderBuilder(GrGLSLProgramBuilder* program, uint8_t fragPosKey);
 
     // Shared GrGLSLFragmentBuilder interface.
     bool enableFeature(GLSLFeature) override;
     virtual SkString ensureFSCoords2D(const GrGLSLTransformedCoordsArray& coords,
                                       int index) override;
     const char* fragmentPosition() override;
 
     // GrGLSLFPFragmentBuilder interface.
+    void appendSampleLocation(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; }
     void onBeforeChildProcEmitCode() override;
     void onAfterChildProcEmitCode() override;
 
     // GrGLSLXPFragmentBuilder interface.
     bool hasCustomColorOutput() const override { return fHasCustomColorOutput; }
     bool hasSecondaryOutput() const override { return fHasSecondaryOutput; }
     const char* dstColor() override;
@@ skipping 64 matching lines @@
     // some state to verify shaders and effects are consistent, this is reset between effects by
     // the program creator
     bool fHasReadDstColor;
     bool fHasReadFragmentPosition;
 
     friend class GrGLSLProgramBuilder;
     friend class GrGLProgramBuilder;
 };
 
 #endif