Use only one callback for every decoder types:

int (*pf_decode)(decoder_t *, block_t *p_block);

There is now only one way to send output frames/blocks from a decoder module:
using decoder_QueueVideo(), decoder_QueueAudio() and decoder_QueueSub()
functions.

This fixes transcoding not receiving any output when a decoder used
decoder_Queue*() function.

The pf_packetize callback is kept unchanged. A packetizer shouldn't be
asynchronous at all (and this simplify the locking for decoder core).

The pf_decode callback returns, for now, only one value: SUCCESS. This will
allow a module to send more status.