Automate graphics conversion

Makefile:

- Don't set DATA, we're no longer embedding files from there directly

- Add folders from GRAPHICS to VPATH. This is how make finds the source files

- Add PNGFILES variable containing all png files from GRAPHICS folders

- Add png files to OFILES with extension changed to rgb.o

- Add rule to create .rgb.o fromn .rgb using normal bin2o macro

- Add rule to create .rgb from .png using imagemagick

source/main.c

- the extension has changed to .rgb so the header is now <filename>_rgb.h and the variables end in _rgb