update ffmpeg (n0.8-18315-geb567a7)
h264.c: remove custom code.  This caused the decoder to show a blank screen when playing back a h264 file in software mode and the AR changed.
ffmpeg: remove unneeded custom code and update config.h
lavr: print out the mix matrix in ff_audio_mix_set_matrix()

This will print the new matrix if it is set after initialization.
lavr: make sure that the mix function is reset even if no mixing will be done

If the matrix reduction ends up with no mixing matrix needed, we need to still reset the mix function accordingly and log the info to the user.
lavr: cosmetics: reindent
lavr: fix matrix reduction for upmixing in certain cases

Do not skip an output if the corresponding input contributes to other output channels.
update ffmpeg/custom_code.txt
no need for one line blocks
remove the unused ID_HELP_DOCUMENTATION