Translation of tile/object coordinates

[bitcraft]

Tiled has a somewhat advanced redering engine that is capable of rendering maps that mix tile sizes. While pytmx is not a rendering project, I mistakenly took liberty to translating the y coordinates of objects from the tiled map coordinate space to the one that pygame typically uses.

Sometime in the future, I will move pytmx to follow Tiled's format more closely. I will break a few specific cases where objects are loaded, but it will make rendering of mixed tile sizes easier and also developing loaders for other graphics libraries.

Since this goal is all very hand-wavy at this point, I'm not assigning a milestone, but will probably make these changes when I include the pyglet loader (in the works!)