examples: Add PNG files and conversion script for bg example

examples/graphics/bg/assets/convert.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+
+grit nfl.png -ftb -fh! -gTFF00FF -gt -gB8 -mR8 -mLs
+grit bg0.png -ftb -fh! -gTFF00FF -gt -gB8 -mR8 -mLs
+grit bg1.png -ftb -fh! -gTFF00FF -gt -gB8 -mR8 -mLs
+grit bg2.png -ftb -fh! -gTFF00FF -gt -gB8 -mR8 -mLs
+grit bg3.png -ftb -fh! -gTFF00FF -gt -gB8 -mR8 -mLs
+
+for file in *.bin; do
+    mv -- "$file" "${file%.bin}"
+done
+
+mv *.pal *.img *.map ../nitrofiles/bg