Vertical position of map object is "artificially" modified

Hey there again!

While working with tmxlib I've noticed my map objects are position incorrectly, especially offseting `y` value by object's height is producing undesired effects. I'm wondering if this is done intentionally or it's a bug?

I've attached a pull request which highlights 2 pieces of code that might affect object positioning, one is the logic that offsets item's `y` value by it's height, second is a piece of code in `mapobject.py` that I can't figure out what is doing.