Use emulated double precision in mandelbrot

I followed the advice of @rossant in #485 to add double precision to the mandelbrot example. I have two questions:

1. Should we have a separate example called `mandelbrot-double.py`? This would be a simple way of making the double precision optional.
2. According to [this](http://www.thasler.org/blog/?p=133) blog post, NVIDIA compilers remove the double precision emulation by default, unless you add some pragmas. I added the pragmas to both shaders, but the problem persists when I run the example on an NVIDIA GPU (it works on my CPU). Could vispy be removing the `#pragma` lines?

I might make the colors smoother later on as well.
Maybe add a docstring to the example stating what the example does, with references? (see other examples)