servo: Merge #14924 - Webgl blend func tests (from ioctaptceb:webgl-blendFunc-tests); r=jdm

<!-- Please describe your changes on the following line: -->
Add a function that validates the conditions for [Constant Color Blend](https://www.khronos.org/registry/webgl/specs/latest/1.0/#CONSTANT_COLOR_BLEND) in the WebGL specs. ./mach run -r tests/wpt/web-platform-tests/webgl/conformance-1.0.3/conformance/misc/webgl-specific.html

---
<!-- 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
- [x] `./mach test-tidy` 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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Source-Repo: https://github.com/servo/servo
Source-Revision: 783829eccc44e1c239a4d1aa807b000b755eeea6

servo/components/script/dom/webglrenderingcontext.rs

@@ -103,6 +103,15 @@ macro_rules! object_binding_to_js_or_null {
     };
 }
 
+fn has_invalid_blend_constants(arg1: u32, arg2: u32) -> bool {
+    match (arg1, arg2) {
+        (constants::CONSTANT_COLOR, constants::CONSTANT_ALPHA) => true,
+        (constants::ONE_MINUS_CONSTANT_COLOR, constants::ONE_MINUS_CONSTANT_ALPHA) => true,
+        (constants::ONE_MINUS_CONSTANT_COLOR, constants::CONSTANT_ALPHA) => true,
+        (constants::CONSTANT_COLOR, constants::ONE_MINUS_CONSTANT_ALPHA) => true,
+        (_, _) => false
+    }
+}
 /// Set of bitflags for texture unpacking (texImage2d, etc...)
 bitflags! {
     #[derive(HeapSizeOf, JSTraceable)]
@@ -808,6 +817,18 @@ impl WebGLRenderingContextMethods for WebGLRenderingContext {
 
     // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
     fn BlendFunc(&self, src_factor: u32, dest_factor: u32) {
+        // From the WebGL 1.0 spec, 6.13: Viewport Depth Range:
+        //
+        //     A call to blendFunc will generate an INVALID_OPERATION error if one of the two
+        //     factors is set to CONSTANT_COLOR or ONE_MINUS_CONSTANT_COLOR and the other to
+        //     CONSTANT_ALPHA or ONE_MINUS_CONSTANT_ALPHA.
+        if has_invalid_blend_constants(src_factor, dest_factor) {
+            return self.webgl_error(InvalidOperation);
+        }
+        if has_invalid_blend_constants(dest_factor, src_factor) {
+            return self.webgl_error(InvalidOperation);
+        }
+
         self.ipc_renderer
             .send(CanvasMsg::WebGL(WebGLCommand::BlendFunc(src_factor, dest_factor)))
             .unwrap();
@@ -815,6 +836,18 @@ impl WebGLRenderingContextMethods for WebGLRenderingContext {
 
     // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
     fn BlendFuncSeparate(&self, src_rgb: u32, dest_rgb: u32, src_alpha: u32, dest_alpha: u32) {
+        // From the WebGL 1.0 spec, 6.13: Viewport Depth Range:
+        //
+        //     A call to blendFuncSeparate will generate an INVALID_OPERATION error if srcRGB is
+        //     set to CONSTANT_COLOR or ONE_MINUS_CONSTANT_COLOR and dstRGB is set to
+        //     CONSTANT_ALPHA or ONE_MINUS_CONSTANT_ALPHA or vice versa.
+        if has_invalid_blend_constants(src_rgb, dest_rgb) {
+            return self.webgl_error(InvalidOperation);
+        }
+        if has_invalid_blend_constants(dest_rgb, src_rgb) {
+            return self.webgl_error(InvalidOperation);
+        }
+
         self.ipc_renderer.send(
             CanvasMsg::WebGL(WebGLCommand::BlendFuncSeparate(src_rgb, dest_rgb, src_alpha, dest_alpha))).unwrap();
     }