Only draw ruler ticks within the current clip region

In Java 8, items drawn outside the horizontal ruler's clip region were showing up overlapping the numbers that should be shown. 

It looks like this could be a bug in Swing, but the workaround is reasonable. This should also be faster, since it's doing less work.

Example screenshot: (only 10m should be visible in the horizontal ruler)
![bad-ruler](https://cloud.githubusercontent.com/assets/374898/3959949/73774dd4-2736-11e4-9f24-040b44e87efc.png)