Add region.check function

region.lua

@@ -1,5 +1,5 @@
-local math, setmetatable, vector
-    = math, setmetatable, vector
+local math, getmetatable, setmetatable, vector
+    = math, getmetatable, setmetatable, vector
 
 local math_min, math_max, math_floor, math_ceil, math_round
     = math.min, math.max, math.floor, math.ceil, math.round
@@ -134,6 +134,10 @@ function region:iter_node_positions()
     end
 end
 
+function region:check()
+    return getmetatable(self) == metatable
+end
+
 function region.equals(a, b)
     return vector.equals(a.min, b.min)
        and vector.equals(a.max, b.max)