Bug 1476368 - Pre-allocate primitives vector in setup_picture_caching. r=gw

--HG--
extra : source : 32aa85626f6c484dff616ec9ae78ca53ea38dec5
extra : histedit_source : 109a92a512932d686f31906c37392f5f55db18e8

gfx/wr/webrender/src/display_list_flattener.rs

@@ -312,10 +312,7 @@ impl<'a> DisplayListFlattener<'a> {
         };
 
         // Get the list of existing primitives in the main stacking context.
-        let mut old_prim_list = mem::replace(
-            primitives,
-            Vec::new(),
-        );
+        let mut old_prim_list = primitives.take();
 
         // In the simple case, there are no preceding or trailing primitives,
         // because everything is anchored to the root scroll node. Handle
@@ -409,6 +406,7 @@ impl<'a> DisplayListFlattener<'a> {
 
         // This contains the tile caching picture, with preceding and
         // trailing primitives outside the main scroll root.
+        primitives.reserve(preceding_prims.len() + trailing_prims.len() + 1);
         primitives.extend(preceding_prims);
         primitives.push(instance);
         primitives.extend(trailing_prims);