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.

Use SleepEx so APC can be executed

__thread is non-standard (and would need mingw 4.3).