[Add] Nebulae battles deserve their own kickass music track.

snd/music.lua

@@ -181,7 +181,16 @@ function choose_combat()
     return
   end
 
+  -- Get some data about the system.
+  sys = space.getSystem()
+  nebu_dens, nebu_vol = sys:nebulae()
+
+  nebu = nebu_dens > 0
+  if nebu then
+    combat = { "nebu_battle1" }
+  else
     combat = { "galacticbattle" }
+  end
 
   music.load(combat[rnd.int(1, #combat)])
   music.play()