Regression introduced in e99cd57 / 4935: VideoBackends: Set the maximum
range when the depth range is oversized[1]. The NV_depth_buffer_float
extension is not part of OpenGL 3.0, and requiring it causes a hard
crash when it's not supported (e.g. macOS).
[1]: https://github.com/dolphin-emu/dolphin/pull/4935
This removes some nonsense in the extension loader where under an ES context we would still pull all function pointers and just continue onward if we
fail to pull one.
Now function pointers are only pulled if the version of GL or ES actually supports that function.