Add gl_SampleMask functionality to fragment builders

Adds methods for overriding and masking a fragment's sample mask.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1690963003

Committed: https://skia.googlesource.com/skia/+/533cefe5b9c7cec2592fc7ca00ee4cf69a26c094

Review URL: https://codereview.chromium.org/1690963003

1 files changed, 6 insertions, 2 deletions

diff --git a/src/gpu/glsl/GrGLSLShaderBuilder.h b/src/gpu/glsl/GrGLSLShaderBuilder.h
index bc3b4ca717..7bfc610bde 100644
--- a/src/gpu/glsl/GrGLSLShaderBuilder.h
+++ b/src/gpu/glsl/GrGLSLShaderBuilder.h
@@ -152,13 +152,17 @@ protected:
         kExternalTexture_GLSLPrivateFeature,
         kFramebufferFetch_GLSLPrivateFeature,
         kNoPerspectiveInterpolation_GLSLPrivateFeature,
-        kLastGLSLPrivateFeature = kNoPerspectiveInterpolation_GLSLPrivateFeature
+        kSampleVariables_GLSLPrivateFeature,
+        kSampleMaskOverrideCoverage_GLSLPrivateFeature,
+        kLastGLSLPrivateFeature = kSampleMaskOverrideCoverage_GLSLPrivateFeature
     };
 
     /*
      * A general function which enables an extension in a shader if the feature bit is not present
+     *
+     * @return true if the feature bit was not yet present, false otherwise.
      */
-    void addFeature(uint32_t featureBit, const char* extensionName);
+    bool addFeature(uint32_t featureBit, const char* extensionName);
 
     enum InterfaceQualifier {
         kOut_InterfaceQualifier,