use geom::num::Zero rather than the deprecated num::Zero trait from std

This fixes the following warning:
```
display_list/mod.rs:735:20: 735:30 warning: use of deprecated item: Use `Int::zero()` or `Float::zero()`., #[warn(deprecated)] on by default
display_list/mod.rs:735         let zero = Zero::zero();
                                           ^~~~~~~~~~
```