This should not be used. It is deprecated and will be removed or ignored when the opengl_cb API is removed.
This retrieves OpenGL function pointers, and will use them in subsequent operation. Usually, you can simply call the GL context APIs from this callback (e.g. glXGetProcAddressARB or wglGetProcAddress), but some APIs do not always return pointers for all standard functions (even if present); in this case you have to compensate by looking up these functions yourself when libmpv wants to resolve them through this callback. libmpv will not normally attempt to resolve GL functions on its own, nor does it link to GL libraries directly.
Value passed as ctx parameter to get_proc_address().
For initializing the mpv OpenGL state via MPV_RENDER_PARAM_OPENGL_INIT_PARAMS.