ffmpeg: simplify mid-stream reconfiguration when libavfilter is disabled

Implement lazy initialization of the image resample context, so that
the initialization code does not need to be duplicated.

Also manage the case in which resample/size change mid-stream. For
each packet the code computes if resampling is needed.

Previously the resample check was done only at the initialization, in
case size/format changed and no resample was detected at the init
stage ffmpeg was silently exiting.
