Until now, stream_peek() read only the bare minimum it had to read from
the stream. But this could cause problems, such as being very
inefficient when peeking a lot, or conflicting with ability to seek
back. (The latter issue can be caused by peeking a few bytes, and then
doing a stream_read() with a size that is 1 byte longer: this would read
the peeked data, then call stream_fill_buffer(), which throws away the
previously peeked bytes - so you can't seek back anymore. This is
mitigated by a hack in demux_open(): it peeks a full buffer, to avoid
that peeking/reading during demuxer probing [or before that, in a stream
filter] can cause the buffer to be dropped.)