swresample: Fix swr_drop_output so it does not flush the buffers

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/af_aresample: split flushing code out

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/af_aresample: Limit data per inserted packet

This avoids creating unwieldy large packets, which is allowed but
does not seem to be a good idea

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>