thumbnailing: speed-up for audio-only media by trusting libvlc on the presense of a video track, tweak libavcodec performance
thumbnailing: speed-up for audio-only media by trusting libvlc on the presense of a video track, tweak libavcodec performance

simon1984/test,hpsoar/VLCKit,programming086/VLCKit,hpsoar/VLCKit,simon1984/test,kamcord/VLCKit,simon1984/test,waterskier2007/VLCKit,waterskier2007/VLCKit,kamcord/VLCKit,siuying/VLCKit,brion/VLCKit,i386/vlckit,larrytin/VLCKit,CollabraMusic/VLCKit,CollabraMusic/VLCKit,siuying/VLCKit,larrytin/VLCKit,CollabraMusic/VLCKit,waterskier2007/VLCKit,strukturag/VLCKit,strukturag/VLCKit,kamcord/VLCKit,programming086/VLCKit,siuying/VLCKit