tile: wrap property doesn't work

[nobuyukinyuu]

wrap_x, wrap_y layer properties don't work when loaded using the tilemap loader. Setting properties to "true" in Tiled does reflect that the internal property for wrapping is updated, but it appears to not be drawing a fully-wrapped map.

http://www.mediafire.com/download/gv747m5hro4gkhm/WrapMap.xml

[swoolcock]

Seems to be something to do with the way I implemented screen boundaries. I'll have to rewrite a large portion of it, but it should be for the better.