Fix loop count from 7b9c8e6

After updating WebRender, [this box shadow][1] rendered incorrectly.
`git bisect` pointed to 7b9c8e6, where I noticed that the replacement
code wasn't quite equivalent -- `self.vertices` vs `vertices`

[1]: https://github.com/kevinmehall/webrender-
experiments/blob/master/src/main.rs#L127

src/batch.rs

@@ -434,7 +434,7 @@ impl RasterBatch {
                                                         dest_rect.size.height as f32)));
                 let mut i = 0;
                 let index_offset = self.vertices.len();
-                while i < index_offset {
+                while i < vertices.len() {
                     let index_base = (index_offset + i) as u16;
                     self.indices.push(index_base + 0);
                     self.indices.push(index_base + 1);