fix: reduce max possible height of terrain hills

From time to time there was tank on the highest hill shown out of
screen.

Now maximum possible height of hills is reduced to keep some space for
tank with its label.

terrain.go

@@ -82,7 +82,8 @@ func GenerateTerrain(g *TerrainGenerator) *Terrain {
 	noise := osx.NewNormalized(g.Seed)
 	heights := make([]int, g.Width)
 	for x := 0; x < g.Width; x++ {
-		heights[x] = int(float64(g.Height) * noise.Eval2(g.Roughness/float64(g.Width)*float64(x), 0.5))
+		// reduce height to keep 5 cells space for tank on the highest hill top
+		heights[x] = 5 + int(float64(g.Height-5)*noise.Eval2(g.Roughness/float64(g.Width)*float64(x), 0.5))
 	}
 	return &Terrain{line: heights, height: g.Height}
 }