Since the pointer `oc' is NULL, oc->oformat->name will cause a null
pointer dereference.  This patch changes it to seg->oformat->name.
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Martin Storsjö <martin@martin.st>

Was left out from c1a02e88.

It is used only in one place and is unlikely it would be needed
elsewhere.

Deprecate avcodec_get_chroma_sub_sample.

Simplifies usage but has higher latency.