Add desaturation effect when low on blood. Fixes #294 and #395.

Also spawn a thread per client (not associated with a particular unit) for processing all medical visual effects. If you switch to a unit low on blood the effect auto updates, and it return to normal when you switch back.

Switched unconciousness effects too and fixed #395.

We could centralize here the pain effects for similar behaviours if they are reimplemented.
Anyone knows how to detect this?