4:2:0 (vertical) chroma subsampling is dependent on interlacing vs.
progressive, and the MPEG decoder decides which to use based on
'progressive-frame' flag, and this flag is set/cleared in the MPEG
stream by 'fieldenc' parameter in the encoder library, so:
o Automagically determine 'fieldenc' (aka "-I n") from the input stream
header's interlacing info (allowing command-line arg to override).
Also:
o Declare "input_interlacing" as 'int', not 'unsigned int'.
o Initialize "input_interlacing" to Y4M_UNKNOWN.
o Promote unknown stream interlacing to 'progressive' for input_interlacing.
