Background red/blue for last played move

This patch makes the last move for each player look different when drawn so it's easier to distinguish them. It makes a new function to do it so the API hasn't changed and the normal show_bd function still works correctly.
![screenshot-2013 02 24 23:10:11](https://f.cloud.github.com/assets/1733611/190888/8ac31dda-7f1a-11e2-8e77-2f27778c44bc.png)