Auto merge of #11324 - dzbarsky:sample_coverage, r=nox

Implement SampleCoverage

Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data:
- [X] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy --faster` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

Either:
- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/script/dom/webglrenderingcontext.rs

@@ -1306,6 +1306,11 @@ impl WebGLRenderingContextMethods for WebGLRenderingContext {
         }
     }
 
+    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
+    fn SampleCoverage(&self, value: f32, invert: bool) {
+        self.ipc_renderer.send(CanvasMsg::WebGL(WebGLCommand::SampleCoverage(value, invert))).unwrap();
+    }
+
     // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.4
     fn Scissor(&self, x: i32, y: i32, width: i32, height: i32) {
         self.ipc_renderer

components/script/dom/webidls/WebGLRenderingContext.webidl

@@ -619,7 +619,7 @@ interface WebGLRenderingContextBase
 
     //void renderbufferStorage(GLenum target, GLenum internalformat,
     //                         GLsizei width, GLsizei height);
-    //void sampleCoverage(GLclampf value, GLboolean invert);
+    void sampleCoverage(GLclampf value, GLboolean invert);
     void scissor(GLint x, GLint y, GLsizei width, GLsizei height);
 
     void shaderSource(WebGLShader? shader, DOMString source);