Bug 1642629 - Reserve items in SegmentBuilder before simple loops. r=kvark

Differential Revision: https://phabricator.services.mozilla.com/D80192

gfx/wr/webrender/src/segment.rs

@@ -331,6 +331,8 @@ impl SegmentBuilder {
             ),
         ];
 
+        self.items.reserve(segments.len() + 1);
+
         for segment in segments {
             self.items.push(Item::new(
                 *segment,
@@ -378,6 +380,8 @@ impl SegmentBuilder {
                         let p2 = inner.bottom_right();
                         let p3 = rect.bottom_right();
 
+                        self.items.reserve(9);
+
                         let corner_segments = &[
                             LayoutRect::new(
                                 LayoutPoint::new(p0.x, p0.y),