Error in Brownian example
When I try to run the Brownian example, I get the following error:

```
* (make-instance 'sketch-examples:brownian)

debugger invoked on a SDL2::SDL-RC-ERROR in thread
#<THREAD "SDL2 Main Thread" RUNNING {1004CF4B43}>:
  SDL Error (#<SDL-WINDOW {#X00000000}>): Couldn't find matching GLX visual
```

I am running SBCL 1.2.14 on Ubuntu 15.10, with Intel HD Graphics 5500. I guess there's a good chance this is something to do with the Intel graphics drivers.

Commenting out [these lines](https://github.com/vydd/sketch/blob/645b42c07d221e7672d464b18eb43a0413860c6c/src/sketch.lisp#L174-L175) seems to solve the problem, but I cannot say I know why.
