Tilemap/VX: Ensure proxy objects don't outlive their parents

Either of these would previously crash (same for VX):

tm = Tilemap.new
at = tm.autotiles
tm = nil
GC.start
at[0] = Bitmap.new(1, 1)

tm = Tilemap.new
at = tm.autotiles
tm.dispose
at[0] = Bitmap.new(1, 1)

Funnily, this makes RMXP itself crash too, but crashing is
never acceptable except for possibly resource exhaustion.

binding-mruby/binding-util.cpp

@@ -41,6 +41,7 @@ struct
 	SYMD(tone),
 	SYMD(rect),
 	SYMD(src_rect),
+	SYMD(tilemap),
 	SYMD(tileset),
 	SYMD(autotiles),
 	SYMD(map_data),