demux_playlist: detect headerless m3u files by extension

m3u files are normally just text files with a list of filenames. Nothing
actually mandates that there is a header. Until now, we've rejected such
files, because there's absolutely no way to detect them.

If nothing else claims the file, the extension is ".m3u", and if the
contents of the file look like text, then load it as m3u playlist. The
text heuristic is pretty cheap, but at least it should prevent trying
to load binary data as playlist. (Which would "work", but result in a
catastrophic user experience.)

Due to the text heuristic, UTF-16/32 files will be rejected (unless they
have a header), but I don't care.
demux: silence unseekable message

This message was added in commit a0acb6ea. But it showed up in all sorts
of inappropriate contexts, such as when opening m3u from an unseekable
http URL, or playing DVDs. So I guess this didn't work out. Disabling it
again.
vaapi/GLX: remove a direct reference to an GL symbol

Instead go through our function loader.
vo_opengl: extend filter size to 32

Also replace the weights calculations for 8/12/16 with the generic
weight function definition macro. (The weights 2/4/6 follow slightly
different rules.)
vo_opengl: disable fancy-downscaling for opengl-hq again

Seems it's actually buggy, beyond the problem that large filter sizes
are required.