opengl: sampler: fix some issues with the libplacebo integration

Warning notice:

Since glad seems to be filling globals regardless of the context provided, I'm not really sure that the code can work well with multiple OpenGL implementation being used in parallel, meaning that the GetProcAddress function might not be called and previous cached values could be used instead, but it has not been investigated.

The problem already existed with libepoxy, so this is only fixing the new issue on tvOS.