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
```