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

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

Packet ptses correspond to frame numbers. This plus doubling 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

Use sample rate as field rate for separate fields layout.
WIP mxfenc: support JPEG2000