The old code generates a termination packet with the same regions as the
start packet and page_state set to "only what changed"; the result is
that the termination packet is decoded as identical to the start packet.

The new code does as found in some DVB broadcasts: produce a packet with
no regions. This is done by expecting num_rects to be 0 rather than
using a flip-flop. ffmpeg.c is updated accordingly.