[libmpv] drop gl_init_params' deprecated parameter for api 2.0
Fixes trivial build issue with upcoming mpv-0.35.0 (libmpv.so.2), aka:
cplusplus/libmpv/mpvobject.cpp:77:89: error: too many initializers for 'mpv_opengl_init_params'
77 | mpv_opengl_init_params gl_init_params{get_proc_address_mpv, nullptr, nullptr};
| ^