webgl: Don't forget to update FBO status after detaching a texture.

We had this in the renderbuffer detach process, but missed it in this
one.

components/script/dom/webglframebuffer.rs

@@ -293,6 +293,7 @@ impl WebGLFramebuffer {
 
             if matched {
                 *attachment.borrow_mut() = None;
+                self.update_status();
             }
         }
     }