Add cel tests

scripts/cel.lua

@@ -0,0 +1,22 @@
+-- Copyright (C) 2018  David Capello
+--
+-- This file is released under the terms of the MIT license.
+-- Read LICENSE.txt for more information.
+
+local s = Sprite(32, 64)
+assert(#s.layers == 1)
+assert(#s.cels == 1)
+
+local c = s.cels[1]
+assert(c.sprite == s)
+assert(c.layer == s.layers[1])
+assert(c.frame == 1)
+assert(c.image)
+assert(c.bounds == Rectangle(0, 0, 32, 64))
+assert(c.color == Color())
+assert(c.data == "")
+
+c.color = Color{ r=255, g=100, b=20 }
+c.data = "test"
+assert(c.color == Color{ r=255, g=100, b=20 })
+assert(c.data == "test")