[rect] Fix variables name.

rect.lua

@@ -51,7 +51,7 @@ function rect.union(x1,y1,w1,h1, x2,y2,w2,h2)
     x1  = min(x1, x2)
     y1  = min(y1, y2)
     xw1 = max(xw1, xw2)
-    xh1 = max(xh1, xh2)
+    yh1 = max(yh1, yh2)
 
     return x1, y1, xw1 - x1, yh1 - y1
 end