This patch cleans up dr_rdpng.h/cc and moves both files to makeobj/ since the code is only used by makeobj.
These fiels are used by heightmaps too, if the heightmap patch gets some more love. Also help files with pngs are clearly wished for. So I do not think moving is the desirable.
The write png part is also used to fump the content of makefiles, see the patches directory. Just standard makeobj is build without to make stealing graphics from non-free paks more difficult.