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.

src/tilemap.h

@@ -55,7 +55,7 @@ public:
 
 	void update();
 
-	Autotiles &getAutotiles() const;
+	Autotiles &getAutotiles();
 	Viewport *getViewport() const;
 
 	DECL_ATTR( Tileset,    Bitmap*   )
@@ -68,6 +68,7 @@ public:
 
 private:
 	TilemapPrivate *p;
+	Autotiles atProxy;
 
 	void releaseResources();
 	const char *klassName() const { return "tilemap"; }