Enable normal-map-specific compression for skins and terrains

Normal maps contain redundant color information and the engine supports
compression algorithms (like DXT5n) optimized for that.
It has previously only been enabled for the water normal maps.

Pull Request: #8114

binaries/data/mods/public/art/textures/textures.xml

@@ -1,4 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Textures>
   <File pattern="*" format="dxt5" mipmap="true" filter="box"/>
+  <File pattern="*_norm*.png" format="dxt5" normal="true" mipmap="true" filter="box"/>
 </Textures>