Now with GMS 2.3 source files!

This is a relatively minor update, since I mostly used the built-in GM compatibility features to convert the project, but there were two breaking changes that needed manual attention.

• Surface handling has changed, so now the surface target is reset before surfaces are freed when rendering samplers.
• Compatibility tile functions are slow as heck and it made the game unplayable when just a handful of enemies were  on-screen. There's now a hack in place which uses a ds_map to store the latest terrain tile generated. This will leak memory each time a new dungeon is generated. Since the rendering is only meant to be a simple placeholder, my reasoning is that you'd want to replace it with your own game objects in the end anyway.