demux_mkv: rewrite packet reading to avoid 1 memcpy()

This directly reads individual mkv sub-packets (block laces) into a
dedicated AVBufferRefs, which can be directly used for creating packets
without a additional copy of the packet data. This also means we switch
parsing of block header fields and lacing metadata to read directly from
the stream, instead of a memory buffer.

This could have been much easier if libavcodec didn't require padding
the packet data with zero bytes. We could just have each packet
reference a slice of the block data. But as it is, the only way to get
padding without a copy is to read the laces into individually allocated
(and padded) memory block, which required a larger rewrite.

This probably makes recovering from broken mkv files slightly worse if
the transport is unseekable. We just read, and then check if we've
overread. But I think that shouldn't be a real concern.

No actual measureable performance change. Potential for some
regressions, as this is quite intrusive, and touches weird obscure shit
like mkv lacing. Still keeping it because I like how it removes some
redundant EBML parsing functions.

demux/ebml.h

@@ -80,9 +80,8 @@ struct ebml_parse_ctx {
 
 bool ebml_is_mkv_level1_id(uint32_t id);
 uint32_t ebml_read_id (stream_t *s);
-uint64_t ebml_read_vlen_uint (bstr *buffer);
-int64_t ebml_read_vlen_int (bstr *buffer);
 uint64_t ebml_read_length (stream_t *s);
+int64_t ebml_read_signed_length(stream_t *s);
 uint64_t ebml_read_uint (stream_t *s);
 int64_t ebml_read_int (stream_t *s);
 int ebml_read_skip(struct mp_log *log, int64_t end, stream_t *s);