Improve PyGame display of tree mutations

[brianzhang01]

Right now the mutations can hit the edge of the tree_canvas, in which case they get cut off. We should make tree_canvas bigger to guarantee the mutations don't get cut off.

Also, we should think about how we want to display multiple mutations on the same branch. Right now we draw over the same mutation which is always placed halfway on the branch.

Additionally, we may want to label or colour mutations so there is a better visual correspondence with the mutations shown along the genome.