Bindings - Export Functions for FFI

[MichealReed]

With #16 we are building the recipe for our libraries but nothing is exported. The task is to handle exports within gpu.h directly for the varying platforms or to provide an additional file with the exports.

[MichealReed]

Better to allow end users to wrap this themselves, may be hard to find a universal wrapper that works for all. You can see my WIP towards a wrapper for dart here.

https://github.com/MichealReed/minigpu/tree/master/minigpu_ffi/src