Add replacement beam sprites (#173)

Add imagemagick script for making bullet sprites

graphics/beam.txt

@@ -0,0 +1,4 @@
+Derived from Bullet Collection 2 (M484 Games)
+by Master484 (http://m484games.ucoz.com/)
+http://opengameart.org/content/bullet-collection-2-m484-games
+http://creativecommons.org/publicdomain/zero/1.0/

graphics/make_bullet_spritesheet.sh

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+#!/bin/sh
+#
+# Create a bullet spritesheet by rotating a source image
+#
+if [ "$#" -ne 1 ]; then
+    echo "Usage: make_bullet_spritesheet.sh infile"
+    exit 1
+fi
+command='convert $1 -background none -virtual-pixel background'
+
+for i in `seq 45 45 315`; do
+  command="$command \\( -clone 0 -filter Point -distort SRT $i \\)"
+done
+
+BASENAME=`basename "$1" | cut -d. -f1`
+DIMENSIONS=`identify -format "%wx%h" $1`
+OUTFILE=${BASENAME}_${DIMENSIONS}.png
+
+command="$command +append -channel A -threshold 60% $OUTFILE"
+
+eval $command
+
+chmod 644 $OUTFILE