Render triangular midpoints to show direction of any selected way

Fixes "Show way direction for any selected road" #1085 
Something I threw together tonight. Not sure if I like it. 
If we draw the midpoints as triangles instead of circles, we can show the direction of any way.  It's subtle.
![screenshot 2014-07-15 23 50 42](https://cloud.githubusercontent.com/assets/38784/3594651/5c682e04-0ca3-11e4-8609-c499e8b73b0f.png)

Before merging, maybe I can also address "Midpoints difficult to see on some paths with patterns" #1448 too.  e.g. They are almost impossible to see on sidewalks.
![screenshot 2014-07-16 00 44 47](https://cloud.githubusercontent.com/assets/38784/3594682/f91b6162-0ca3-11e4-8429-356b2995b1cc.png)