Convert banners to png as gdk-pixbuf-csource no longer supports bmp
I updated the EDuke32 flatpak to the most recent freedesktop platform and it wouldn't compile as gdk-pixbuf no longer includes BMP support by default.
Rather than adding the png files I could add the following to the makefile
%.png: %.bmp
ffmpeg -i $< -frames:v 1 -update 1 $@
Figuring out which software to use to convert the images seemed more complex than just uploading the pngs.
The bmp files are still used by source/sw/rsrc/gameres.rc
etc. in the form RSRC_BMP BITMAP "build.bmp"
Edited by Colin Kinloch