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-mri/tilemap-binding.cpp

@@ -94,6 +94,10 @@ RB_METHOD(tilemapInitialize)
 
 	rb_iv_set(autotilesObj, "array", ary);
 
+	/* Circular reference so both objects are always
+	 * alive at the same time */
+	rb_iv_set(autotilesObj, "tilemap", self);
+
 	return self;
 }
 

binding-mri/tilemapvx-binding.cpp

@@ -64,6 +64,10 @@ RB_METHOD(tilemapVXInitialize)
 
 	rb_iv_set(autotilesObj, "array", ary);
 
+	/* Circular reference so both objects are always
+	 * alive at the same time */
+	rb_iv_set(autotilesObj, "tilemap", self);
+
 	return self;
 }