Drive libdvdnav's menu state through the new disc-nav protocol so the
player can show and interact with VMGM and VTSM menus.
STREAM_CTRL_NAV_CMD maps the nav actions onto libdvdnav.
STREAM_CTRL_GET_NAV_STATE reports whether a menu with buttons is on
screen, the highlight rectangle and palette of the focused button, and
the ids of the disc-selected audio/SPU streams.
Menu mode is entered by opening with dvd://menu or --disc-menu, which
starts at the disc's root menu instead of a title.
Kacper Michajłow
b17feb1f79