cntr.c could use an update

The code in [`chaco/contour/cntr.c`](https://github.com/enthought/chaco/blob/master/chaco/contour/cntr.c) is outdated and a little buggy (as I discovered when I tried to pass the optional mask to the `Cntr` type). It appears to be a clone of [this code](https://github.com/matplotlib/matplotlib/blob/master/src/cntr.c) in matplotlib, which looks a little more up-to-date.