fix upipe_av_samplefmt_to_flow_def
fix a52 framer
fix swresample channel mixing
be more tolerant with MAX_PCR_INTERVAL
remove useless log
new upipe_suggest_flow_def API

This allows to ask an encoder to modify a proposed flow def so that the
flow gets compatible with the encoder. You would then typically use
swscale or swresample to convert the flow def to the new proposed def.