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/packet.c

@@ -75,6 +75,17 @@ struct demux_packet *new_demux_packet_from_avpacket(struct AVPacket *avpkt)
     return dp;
 }
 
+// (buf must include proper padding)
+struct demux_packet *new_demux_packet_from_buf(struct AVBufferRef *buf)
+{
+    AVPacket pkt = {
+        .size = buf->size,
+        .data = buf->data,
+        .buf = buf,
+    };
+    return new_demux_packet_from_avpacket(&pkt);
+}
+
 // Input data doesn't need to be padded.
 struct demux_packet *new_demux_packet_from(void *data, size_t len)
 {