Remove Need to Specify Actors in `Makefile.split`

Having to edit the Makefile.split when adding custom textures is very annoying and super easy to forget about. For anybody that doesn't already know about the needed `Makefile.split` edits, it can serve as a full roadblock with no obvious solutions or alternatives, hence high priority.

Also allow support for PNG assets in custom subdirectories. (e.g. adding a custom subdirectory with images to one of the level folders will currently cause them to go unseen by the compiler.)