The loader now doesn't attempt to check for extensions and just loads the
function pointers. It was doing that for all core functions anyway, so a
few more DSA functions won't hurt the loading times too much. Should fix
issues with core-only contexts on OSX.
Caused linker problems mainly on Windows, where every symbol would need
to be exported manually, on Linux this was done implicitly (or with
`-fvisibility=default`). I now maintain glLoadGen fork at
https://github.com/mosra/glloadgen.git containing changes needed for
Magnum.
The `ogl_*()` functions are used only internally, various global
variables for extension queries are not used at all and thus they don't
need to be exported. I thus enabled `-fvisibility=hidden`.
Fixes#16.
Vladimír Vondruš