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().

(Eventually, this could replace block_FifoWake))

Every thread function disables cancellation properly now.