Add arb_texture_rectangle support properly in GLES3 shaders

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 e998458158af65115066d10c521afca711dd1f5b..3e9fe0e539c7279008b16512da32f31587077ac9 100644
--- a/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
+++ b/src/gpu/glsl/GrGLSLFragmentShaderBuilder.h
@@ -169,14 +169,16 @@ private:
     void onFinalize() override;
 
     /**
-     * Features that should only be enabled by GrGLSLFragmentShaderBuilder itself.
+     * Features that should only be enabled by GrGLSLFragmentShaderBuilder itself
+     * or its subclass.
      */
     enum GLSLPrivateFeature {
         kFragCoordConventions_GLSLPrivateFeature = kLastGLSLFeature + 1,
         kBlendEquationAdvanced_GLSLPrivateFeature,
         kBlendFuncExtended_GLSLPrivateFeature,
         kExternalTexture_GLSLPrivateFeature,
-        kLastGLSLPrivateFeature = kBlendFuncExtended_GLSLPrivateFeature
+        kRectangleTexture_GLSLPrivateFeature,
+        kLastGLSLPrivateFeature = kRectangleTexture_GLSLPrivateFeature
     };
 
     // Interpretation of FragPosKey when generating code