[RFC] HEVC muxing in Matroska/MPEG-4.

I believe/hope I managed to get hvcC creation right
(as per ISO/IEC 14496-15). -vcodec copy output seems
to work in VLC. -vcodec libx265 doesn't work because
the libx265 wrapper doesn't set the context extradata.

I didn't bother adding 3gp/f4v tags for HEVC, as I don't
know whether this is even supported/possible yet.

Similarly, I don't know whether mov_write_isml_manifest()
can and/or should be updated yet.

I didn't update mov_write_ftyp_tag() either (same reasons).

I tried to update the AVC-specific code in matroskaenc
and movenc when it looked like it may have been applicable
to HEVC as well. In some cases, I have no clue what that
code actually does, so it may be quite wrong. Examples:

/* nal reformating needed */ in movenc.c

ff_avc_parse_nal_units_buf(pkt->data, &data, &size);
in matroskaenc.c, only when a start code is found in
the extradata (Annex B formatted), but not if the
extradata is e.g. already hvcC-formatted.

ff_isom_write_avcc() and
ff_avc_write_annexb_extradata() are H.264-specific,
but the other ff_avc_*() functions in avc.c are also
applicable to HEVC, thus should probably be renamed
and moved to a separate file. Someone suggested
ff_avc -> ff_xvc, thoughts?