examples: Add assets and conversion script for bg8bit example

examples/_original_assets/nebulae.txt

@@ -0,0 +1,9 @@
+Link: https://unsplash.com/photos/qtRF_RxCAo0
+Author: Aldebaran S
+License: Unsplash
+
+Newton, MA
+Published on January 10, 2021
+WOGT71/ZWO, 60+ frames, ASI1600mm pro
+
+North American and Pelican nebulae mosaic

examples/_original_assets/sea.txt

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+Link: https://unsplash.com/photos/oQl0eVYd_n8
+Author: Joseph Barrientos
+License: Unsplash
+
+Tel Aviv-Yafo, Israel
+Published on August 12, 2015
+SONY, ILCE-7

examples/graphics/bg8bits/assets/convert.sh

@@ -1,2 +1,22 @@
 #!/bin/sh
-grit img8b_3.jpg -gb -gB8 -ftb
+
+# The two backgrounds that share the palette have been edited to use 128 colors
+# each, so that the final combined palette is 256 colors in size.
+
+grit img8b_1.png img8b_2.png -ftb -fh! -gTFF00FF -gb -gB8 -pS -Oimg8b_1.pal
+
+# This image has been edited so that it uses 255 colors and the last one is
+# free. That way we can set it to red manually and paint using it.
+
+grit img8b_3.png -ftb -fh! -gTFF00FF -gb -gB8
+
+for file in *.bin; do
+    mv -- "$file" "${file%.bin}"
+done
+
+# This is required or the library will say that the second background doesn't
+# have a palette.
+
+cp img8b_1.pal img8b_2.pal
+
+mv *.pal *.img ../nitrofiles/bmp