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 _____

Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process.

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="35" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/11324)
<!-- Reviewable:end -->

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);

components/servo/Cargo.lock

@@ -2524,7 +2524,7 @@ dependencies = [
 [[package]]
 name = "webrender_traits"
 version = "0.1.0"
-source = "git+https://github.com/servo/webrender_traits#7a51247cb5333d7a4560e00fe4518db328a550da"
+source = "git+https://github.com/servo/webrender_traits#f74a744614b4c0c2fc64d39916386848124f4d52"
 dependencies = [
  "app_units 0.2.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "byteorder 0.5.2 (registry+https://github.com/rust-lang/crates.io-index)",