Auto merge of #20754 - anholt:webgl-fixes, r=emilio

Webgl fixes

<!-- Please describe your changes on the following line: -->
Add gl.getParameter() support for webgl unpack settings, and remove some unreachable and incorrect error-generation code

---
<!-- 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
- [x] These changes fix #20372
- [x] These changes fix #20553
- [x] These changes fix #20554

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

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components/script/dom/webglrenderingcontext.rs

@@ -1326,6 +1326,14 @@ impl WebGLRenderingContextMethods for WebGLRenderingContext {
                 "WebGL GLSL ES 1.0".to_jsval(cx, rval.handle_mut());
                 return rval.get();
             }
+            constants::UNPACK_FLIP_Y_WEBGL => {
+                let unpack = self.texture_unpacking_settings.get();
+                return BooleanValue(unpack.contains(TextureUnpacking::FLIP_Y_AXIS));
+            }
+            constants::UNPACK_PREMULTIPLY_ALPHA_WEBGL => {
+                let unpack = self.texture_unpacking_settings.get();
+                return BooleanValue(unpack.contains(TextureUnpacking::PREMULTIPLY_ALPHA));
+            }
             _ => {}
         }
 
@@ -1412,13 +1420,7 @@ impl WebGLRenderingContextMethods for WebGLRenderingContext {
         let (sender, receiver) = webgl_channel().unwrap();
         self.send_command(WebGLCommand::GetTexParameter(target, pname, sender));
 
-        match receiver.recv().unwrap() {
-            value if value != 0 => Int32Value(value),
-            _ => {
-                self.webgl_error(InvalidEnum);
-                NullValue()
-            }
-        }
+        Int32Value(receiver.recv().unwrap())
     }
 
     // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3