This is a simple python script that generates a map image of a world based on the already-generated in-game maps. Because this does not depend on the region file format, this should work in 1.12 and 1.13, and probably previous versions as well.

You'll need to install Python. Then you will need the NBT library by twoolie. The easiest way to get this is to use PIP (pip install NBT).

Then, put mapstitcher.py and config.py into a folder. Edit the values in config.py to your liking, then run python mapstitcher.py. That's it!

You can see the different levels of detail that come with different maps being zoomed out to different zoom levels.