Clear out the background when building a trimmed tileset.

Otherwise it's filled with some random color (palette entry
0?) and there are splotches of that filling the gaps between
trimmed tiles.

src/main/java/com/threerings/media/tile/util/TileSetTrimmer.java

@@ -21,7 +21,9 @@
 
 package com.threerings.media.tile.util;
 
+import java.awt.AlphaComposite;
 import java.awt.Dimension;
+import java.awt.Graphics2D;
 import java.awt.Rectangle;
 import java.awt.geom.Area;
 import java.awt.image.BufferedImage;
@@ -165,6 +167,11 @@ public static void trimTileSet (
             image = ImageUtil.createCompatibleImage(source.getRawTileSetImage(),
                 bounds.width, bounds.height);
 
+            // Empty it out
+            Graphics2D graphics = (Graphics2D)image.getGraphics();
+            graphics.setComposite(AlphaComposite.Clear);
+            graphics.fillRect(0, 0, bounds.width, bounds.height);
+
         } catch (RasterFormatException rfe) {
             throw new IOException("Failed to create trimmed tileset image " +
                 "[bounds=" + bounds + ", tset=" + source + ", rfe=" + rfe + "].");