Add upipe-avcdv code (incomplete)

TODO:
- handle flowdefs
- use udeal (be thread safe)
- replace hardcoded frame/ubuf_pic mapping
- replace malloc with some ubuf_* mechanism
Add upipe-avcdv make rules
upipe_avcdv: add (partial) udeal support

TODO (udeal): block source if udeal pump running,
keep uref (list ?), and process/resume afterwards
upipe_avcdv: thread-safe (complete udeal support)
upipe_avcdv: add input flow definition support

Currently using upipe-av flow def strings.
upipe_avcdv: fix no-pump _set_context()

Call upipe_avcdv_use() in _set_context() even with no callback,
since _open_codec() calls upipe_release().
This way _release() can return safely in case of (context).
test/Makefile: avformat/avcodec depend on upump

avformat and avcodec tests currently depend on upump-ev