Auto merge of #17406 - Gankro:with_cap, r=jdm

use DisplayListBuilder::with_capacity

DO NOT MERGE until servo/webrender#1399 has landed in Servo.

Previously webrender was reserving 1MB of space for each display list (empirically pages weighed between 100k and 10MB when I implemented this). This was problematic for gecko because they used webrender differently. This just brings Servo back to where it was.

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components/layout/webrender_helpers.rs

@@ -223,8 +223,9 @@ impl WebRenderDisplayListConverter for DisplayList {
     fn convert_to_webrender(&self, pipeline_id: PipelineId) -> DisplayListBuilder {
         let traversal = DisplayListTraversal::new(self);
         let webrender_pipeline_id = pipeline_id.to_webrender();
-        let mut builder = DisplayListBuilder::new(webrender_pipeline_id,
-                                                  self.bounds().size.to_sizef());
+        let mut builder = DisplayListBuilder::with_capacity(webrender_pipeline_id,
+                                                            self.bounds().size.to_sizef(),
+                                                            1024 * 1024); // 1 MB of space
 
         let mut current_scroll_root_id = ClipId::root_scroll_node(webrender_pipeline_id);
         builder.push_clip_id(current_scroll_root_id);