invaders: change enemy distribution

This commit changes the distribution of enemies so that each row
contains only one type of enemy.

lib/invaders.lua.in

@@ -195,7 +195,7 @@ function enemies.setup()
     for y = 1, gamedata.enemies.rows do
         gamedata.enemies.data[y] = { }
         for x = 1, math.ceil((gamedata.enemies.count / gamedata.enemies.rows) + 1) do
-            gamedata.enemies.data[y][x] = enemies.new( math.random(1, 3) )
+            gamedata.enemies.data[y][x] = enemies.new((y % 3) + 1)
         end
     end