add a way to tweak and fix lighting on some maps

[Big-Onche]

as qreeves mentioned on Reddit "I recommend creating a mod specific entity file or something (like how waypoints work) so you can actually tweak maps just for the mod. I used to do this back in the day."

Creating a light file alongside each map to load light tweaks without modifying the map:

• loop through each light entity
• write light coordinates (float x,y,z) and attributes (int rad, r, g, b, type)
• create this file when saving a map

When loading a map:

• try to read the lights file while loading a map and put the new lights instead
• use the map's default lights if no or invalid file

[Big-Onche]

done! https://github.com/Big-Onche/Tesseract-Sauerbraten/commit/5c0bb1c291622c622f3cd68db9707de1719b86ab