mpvobject.cpp: use new libmpv (2.0) api
The extra_exts member has been deprecated for years and is now removed.
https://github.com/mpv-player/mpv/commit/632059816adde518c67f6606e3cfdef5bec18b52 https://github.com/mpv-player/mpv/blob/master/libmpv/render_gl.h#L106
Signed-off-by: Alfred Persson Forsberg cat@catcream.org