GPU FRACTAL RENDERER

While I've definitely made fractal renderers before, they'd all been
CPU-based.  Since I'm learning Vulkan right now I figured it would be
a good time to give a GPU-based one a try.  I'm glad I did because it
was very fun indeed.  It uses GLFW and Vulkan, so you'll need the
development libraries for these.  You'll also need a GPU that supports
double precision shaders.

To build and run:

   ./build.sh && ./gpu-frac

Since both GLSL and C use CPP directives, I parameterised the program
a bunch and stuck a load of cool presets in config.h, which can be
simply be uncommented and commented as is required.  All the #ifdefs
make the code a bit harder to read but it's nice being able to twiddle
numbers and change the visualisation without messing around editing
and commenting out code.