examples: Add assets and scripts to more examples

examples/graphics/backbuffer2/assets/convert.sh

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+#!/bin/sh
+
+grit bitmap16.png -ftb -fh! -gb -gB16
+
+for file in *.bin; do
+    mv -- "$file" "${file%.bin}"
+done
+
+mv *.img ../nitrofiles/bmp

examples/graphics/backbuffer4/assets/convert.sh

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+#!/bin/sh
+
+grit bitmap16.png -ftb -fh! -gb -gB16
+
+for file in *.bin; do
+    mv -- "$file" "${file%.bin}"
+done
+
+mv *.img ../nitrofiles/bmp

examples/graphics/imgdraw/assets/convert.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+
+grit bitmap16.png -ftb -fh! -gb -gB16
+grit blueball.png -ftb -fh! -gb -gB16
+
+for file in *.bin; do
+    mv -- "$file" "${file%.bin}"
+done
+
+mv *.img ../nitrofiles/bmp