avformat/apngdec: transmit all the chunks between consecutive fcTL ones.

In order to support multiple IDAT of fdAT chunks following an fcTL one,
transmit all the chunks between two fcTL ones (or between fcTL and IEND
one).

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
doc/examples/decoding_encoding: fix storing all channels

Fixes Ticket3355

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>