stream: de-inline some larger functions

Tests with demux_mkv show that the speed doesn't change (or actually,
it seems to be faster after this change). In any case, there is not
the slightest reason why these should be inline. Functions for which
this will (probably) actually matter, like stream_read_char, are
still left inline.

This was tested with demux_mkv's indexing. For broken files without
index, demux_mkv creates an on-the-fly index. If you seek to a later
part of the file, all data has to be read and parsed until the wanted
position is found. This means demux_mkv will do mostly I/O, calling
stream_read_char() and stream_read(). This should be the most I/O
intensive non-deprecated part of mpv that uses the stream interface.
(demux_lavf has its own buffering.)
demux_lavf: print how many bytes are read by avformat_find_stream_info()

Can be helpful for debugging slow stream startup.

Also add a comment about BIO_BUFFER_SIZE.
stream: kill memory streams

These are unused by now, and it relied on some assumptions that have
been broken by now.
stream: kill STREAM_CTRL_RESET

Was unused.
vf_delogo: copy in non-direct case, fix double-free

If the image is not writeable, the image actually has to be copied
beforehand. This was overlooked when converting the video chain to
reference counted images.

Fix a double free issue. This was overlooked when vf.c was changed to
free filter priv data automatically.
vo_opengl: remove most of the "inline" documentation

There's no point duplicating all the text that is already in the man
pages, and synchronizing them is a pain. Place a link to the github
generated pages instead.

Unfortunately, the anchor '#vo-opengl' does not work. Maybe github's
rst converter just sucks, as the actually generated HTML contains
links using that anchor too, but does not generate the anchor itself.
Too bad.
manpage: mark some filters as deprecated in favor of libavfilter

This is basically a "do not use" label. We don't remove them yet,
because we still support FFmpeg releases where we can not use
libavfilter for various reasons. Also, Libav causes pain as usual
due to the lack of ported mplayer filters in its codebase, so not
all filters will be available there.