lavf/mxfdec: fix codec and stream time base for separate fields layout

Since two packets are required to decoded a frame, we need to divide by
a factor of 2 the stream time base so output ptses are correct.
lavf/mxfdec: generate ptses for interlaced j2k streams

Packet ptses correspond to frame numbers. This plus dividing by a factor
of 2 the stream time base will output correct ptses for decoded frames
since we need 2 packets for one decoded frame.
lavf/mxfdec: add -use_field_rate option

Interpret sample rate as field rate for separate fields layout.