Auto merge of #14818 - glennw:zb, r=pcwalton

Update WR (first phase of z-buffer optimizations / improvements).

<!-- Reviewable:start -->
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/14818)
<!-- Reviewable:end -->

components/servo/lib.rs

@@ -174,9 +174,9 @@ impl<Window> Browser<Window> where Window: WindowMethods + 'static {
                 enable_scrollbars: opts.output_file.is_none(),
                 renderer_kind: renderer_kind,
                 enable_subpixel_aa: opts.enable_subpixel_text_antialiasing,
-                clear_empty_tiles: true,
                 clear_framebuffer: true,
                 clear_color: webrender_traits::ColorF::new(1.0, 1.0, 1.0, 1.0),
+                render_target_debug: false,
             })
         };