added gfx performance test

Fixed #300 

The results to not look great
(all were done with rustc `5419b2ca2`)

**Drawing 1 000 000 triangles**
(System 3930K w/ 780gtx, Ubuntu 14.04, nvidia 331.38 drivers)
```
gl  | ~105ms
gfx | ~3275ms
```

**Drawing 99 856 triangles**
(MBA 1.7Ghz, OS X 10.10)
```
gl  | ~70ms
gfx | ~500ms
```

I'm not sure if the OpenGL is the most fair render to compare it to. It is running with the expectation that nothing needs to be state changed next to the uniform. Which while true for this example is not universally true.