with libdv's mishandling of PALDV. Also remove the YUV4 references. Quicktime
says YUV4 is a 4:2:0 type but lav_io.c was treating it as 4:2:2 (and nothing
known today produced YUV4 anyhow - it's marked as obsolete/invalide in
Quicktime). Also cleanup a little by removing the 'check_YUV420_input' function
and single reference - that actually did NOT check if the input was 4;2:0 at
all! All that did was make sure a frame buffer could be malloc'd - not worth
the extra code since a ~500KB malloc failure will be noticed very quickly.
