hxxx_nal: add bs_t emu prev 3 bytes stripper
hxxx_nal: add bs_t emu prev 3 bytes stripper

Will allow to drop all ep3b/malloc around

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc