Tolerate unknown SEI and expose SEI payloads on decoded pictures

[kpchoi]

Two decoder fixes plus the counterpart of the xeve SEI injection API:

• Unknown SEI payload types made the whole decode fail
  (default: xevd_assert_rv(0, XEVD_ERR_UNEXPECTED)), so any stream with a
  standard SEI such as mastering display colour volume was rejected. They are
  now parsed past, as the spec requires.
• The SEI parser only handled one sei_message() per SEI NAL unit; it now
  loops until the trailing byte.
• SEI payloads of an access unit (everything except the internal picture
  signature) are exposed on the output picture: when
  imgb->ndata[XEVD_IMGB_SEI_SLOT] == XEVD_SEI_MAGIC,
  imgb->pdata[XEVD_IMGB_SEI_SLOT] points to an XEVD_SEI owned by the
  library, valid until the imgb is released. The spare ndata/pdata fields are
  used, so the ABI is unchanged. Slot 3 of the input bitstream buffer is no
  longer propagated to output pictures (it is reserved for this).

The README gains a programming guide for the library including SEI reading,
plus small fixes.

Verified with a round trip against mpeg5/xeve#169: per-frame T.35 + MDCV
payloads with bframes=15 arrive frame-accurate in display order; decode
output of normal streams is unchanged; MAIN/BASE and x86/aarch64 builds pass;
AddressSanitizer run over the whole decode path is clean (including picture
buffer recycling).

[mpeg5]

LGTM