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/tilemapvx.h

@@ -54,7 +54,7 @@ public:
 
 	void update();
 
-	BitmapArray &getBitmapArray() const;
+	BitmapArray &getBitmapArray();
 
 	DECL_ATTR( Viewport,   Viewport* )
 	DECL_ATTR( MapData,    Table*    )
@@ -66,6 +66,7 @@ public:
 
 private:
 	TilemapVXPrivate *p;
+	BitmapArray bmProxy;
 
 	void releaseResources();
 	const char *klassName() const { return "tilemap"; }