Unconst all the FFI calls.

This was done by https://github.com/bjz/glfw-rs/pull/182. However, the
original FFI calls were all incorrectly using `*`. They now match the C
headers: `*mut T` is used when C uses `*T` and `*const T` is used when C
uses `const T *`.
Merge pull request #191 from metajack/un-const

Unconst all the FFI calls.