Remove the DOMToTexture feature

This relies on WebRender's frame output API, `set_output_image_handler`,
which has been removed from the latest upstream [1]. It's sad to remove
this feature, which was probably a lot of work to implement, but it
seems difficult to patch WebRender to restore this functionality.

Fixes #29936.

1. https://hg.mozilla.org/mozilla-central/rev/361521e3c52324809553c555fb066d50f023d9bf

components/servo/lib.rs

@@ -440,10 +440,8 @@ where
             webgl_threads,
             webxr_layer_grand_manager,
             image_handler,
-            output_handler,
         } = WebGLComm::new(
             webrender_surfman.clone(),
-            webrender_gl.clone(),
             webrender_api.create_sender(),
             webrender_document,
             external_images.clone(),
@@ -453,11 +451,6 @@ where
         // Set webrender external image handler for WebGL textures
         external_image_handlers.set_handler(image_handler, WebrenderImageHandlerType::WebGL);
 
-        // Set DOM to texture handler, if enabled.
-        if let Some(output_handler) = output_handler {
-            webrender.set_output_image_handler(output_handler);
-        }
-
         // Create the WebXR main thread
         let mut webxr_main_thread =
             webxr::MainThreadRegistry::new(event_loop_waker, webxr_layer_grand_manager)