Fixed a bug in export for Godot.

Fixed Godot export bug. Previously path to texture in tileset was absolute, so if you copy your game executable to another computer, it was likely to break.

Now on exporting for Godot all directories are scanned for project.godot and if found, start their path to texture from that folder, like any other godot resource (like "res://..."). Note that you need to save generated texture beforehand and it also needs to be inside any godot project. For now the tool does not check if it's the same project with tileset resource.