Auto merge of #26143 - jdm:transform-feedback, r=nox

Fix webgl transform feedback

This allows https://www.joshmatthews.net/webxr-particles/ to run in Servo. This change corresponds with the following spec text:
```
GL_INVALID_OPERATION is generated by glBeginTransformFeedback if no binding points would be used, either because no program object is active of because the active program object has specified no varying variables to record.
```

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #26137
- [x] There are tests for these changes

components/script/dom/webgl2renderingcontext.rs

@@ -3456,7 +3456,7 @@ impl WebGL2RenderingContextMethods for WebGL2RenderingContext {
                 return;
             },
         };
-        if !program.is_linked() || program.transform_feedback_varyings_length() != 0 {
+        if !program.is_linked() || program.transform_feedback_varyings_length() == 0 {
             self.base.webgl_error(InvalidOperation);
             return;
         };