Add Sudden Death counterparts for sky color, sky images and horizont images

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In Hedgewars themes, the sky does not have “real” Sudden Death 
counterparts. Hedgewars just makes the sky color, the sky images and horizont 
images a bit darker and that’s it.

I therefore suggest to introduce Sudden Death counterparts for all of them.

Here are the details:

For theme.cfg:
Add the variable “sd-sky”: Same as sky, but used in Sudden Death. If 
unspecified, its like sky but darker (like now).

For the files:
Add SDSky.png, SDSkyL.png and SDSkyR.png:
Sudden Death counter parts of Sky.png, SkyL.png and SkyR.png. If, and only if 
ALL of these SDSky* images are unspecified, Hedgewars just uses the 
“normal” sky images, but a bit darker (like now).
Please note that the 3 non-SD images and 3 the SD images both as independent 
sky sets. Hopefully you will see clearer with my examples.

Examples:
- Theme uses Sky.png. → Normal: Sky.png repeated. Sudden Death: Sky.png (dark 
version) repeated.
- Theme uses Sky.png, SDSky.png. → Normal: Sky.png repeated. Sudden Death: 
SDSky.png repeated.
- Theme uses Sky.png, SkyL.png. → Normal: Sky.png in center, left and right 
SkyL.png repeated. Sudden Death: same as normal but darker.
- Theme uses Sky.png, SDSky.png, SDSkyL.png, SDSkyR.png. → Normal: Sky.png 
repeated. Sudden Death: SDSky.png in center, left from center SDSkyL.png 
repeated, right from center SDSkyR.png repeated.

And then add SDhorizont.png, SDhorizontL.png and SDhorizontR.png. That’s 
analog to SDSky.png etc., but for horizont.

Original issue reported on code.google.com by almikes@aol.com on 24 Dec 2014 at 4:50