Move all engine stuff to GoRetro. Still many interactions between game and engine code that there should not be.

util_maths.go

@@ -0,0 +1,12 @@
+package GoRetro
+
+func absInt(x int) int {
+	return absDiffInt(x, 0)
+}
+
+func absDiffInt(x, y int) int {
+	if x < y {
+		return y - x
+	}
+	return x - y
+}