For now just create a single PNG for the atlas with a custom text chunk
with the atlas information in GDX’s legacy format, the one read by
Heaps.

The idea is to be able to build the pages independently and then with a
different program “extract” these text files and generate a single
.atlas file.  That way i can have Makefile rules that build a single
.atlas from many SVG and ony rebuild the PNG that actually matter, not
everything.