Update test_mpv to use a dummy Xvfb implementation and gpu renderer to allow it to run on windows.
Bump DLL name to mpv-2.dll and update error message may be reasonable to add in a fallback to mpv-1.dll too but the bleeding edge builds are all following that naming convention.
Remove mpv_opengl_cb_* function references and associated wrappers and function definitions.
Remove mpv_get_sub_api reference and return wrapper.
mpv_opengl_cb_*
function references and associated wrappers and function definitions.mpv_get_sub_api
reference and return wrapper.