Was missing from [629f5752]

vlc_join will cause the assertion failure with pthread.
On -unchanged- Windows, it (supposedly) deadlocks as it used to.

The only user is LUA, inlined there. The return value seems suspicious
by the way.

(Hopefully-would-have-been-Signed-off-by: Pierre d'Herbemont)

It fixes aspect ratio of saved picture (and internal ones).
It fixes aspect ratio of displayed PIP (with and without overlay).
It factorizes a bit the code.
It adds a few checks on malloc.

If we ask to convert but this conversion does nothing (same chroma and
same width/height, swscaler does not copy the pixels.

Simpler and free from arbitrary wake ups

This removes the need for suxor_thread_t.

It fixes SVQ1 (at least).

I have some doubt about it, either swscale does not respect the ffmpeg doc
or the doc is wrong (about the rgb mask).