Use `gl_loader` to load GL rather than directly loading `libGL.so`
From https://github.com/NVIDIA/libglvnd/issues/62#issuecomment-179938512
According to the spec, libGL.so is only supposed to export OpenGL 1.2 and GL_ARB_multitexture, and apps are supposed to get at everything else with glXGetProcAddress.
Use something made for loading GL instead of doing it a janky/incorrect way. Should fix #87 (closed)