Feature vbo mesh draw instanced example

This demonstrates how to use ofVboMesh::drawInstanced() using the programmable GL renderer and a matching shader.

It will draw 16384 instances of an ofBox primitive, spreading these out on a grid, extruding their height on the vertex shader using a depth texture, and animating along the x-axis, to make it a little more interesting.

Reference renders:

![screen shot 2013-07-30 at 11 21 32](https://f.cloud.github.com/assets/423509/878100/01a2fdde-f906-11e2-8ed6-48621f4595cf.png)
![screen shot 2013-07-30 at 11 21 14](https://f.cloud.github.com/assets/423509/878104/0fab5c96-f906-11e2-90ad-405a0af3e0c3.png)


Plenty of comments inline.