Detect changes to clipRect without needing to reset the field in order to take effect

[DetectiveBaldi]

currently, using FlxRect for fields such as FlxSprite.clipRect requires reassigning in order for changes to take effect. i've seen multiple people question this, and i've wondered if there's a simpler way around it.

Some ideas:

• Create a class similar to FlxCallbackPoint (potentially FlxCallbackRect) that acts similarly to the callback point class.
• Implement this functionality in FlxRect itself, possibly a callback function that gets called when setting x, y, width, height.

If we went with the latter, we could potentially remove FlxCallbackPoint and add similar functionality into the standard FlxPoint class.

[Geokureli]

As I said in chat, I'm not a fan of FlxCallbackPoints for memory reasons and I'm even trying to do away with that in the long run. but I think there may be another way to detect changes to the clipRect field, i'll do some test when I have time