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).

This fixes the skin loading. The color seems wrong but I think the skin
code is right (vlc RGBA is stored in memory R, B, G, A).

Better than an unlikely abort().