Animation: implement necessary bits for normalForm()

This allows a layered image to specify an animation as one of its
layers, and for it to still be recognized as an animation after
being processed by MakeTiles.

src/micropolisj/build_tool/MakeTiles.java

@@ -182,6 +182,8 @@ public class MakeTiles
 				continue;
 			}
 
+			ref = ref.normalForm();
+
 			TileImage dest = prepareFrames(ref, c);
 			TileMapping m = new TileMapping(tileName, ref, dest);
 			mappings.add(m);

src/micropolisj/graphics/Animation.java

@@ -7,7 +7,7 @@ import java.util.*;
 import javax.xml.stream.*;
 import static micropolisj.XML_Helper.*;
 
-public class Animation extends TileImage
+public class Animation extends TileImage implements TileImage.MultiPart
 {
 	static final int DEFAULT_DURATION = 125;
 
@@ -52,6 +52,30 @@ public class Animation extends TileImage
 		return a;
 	}
 
+	//implements MultiPart
+	public MultiPart makeEmptyCopy()
+	{
+		return new Animation();
+	}
+
+	//implements MultiPart
+	public Iterable<? extends Part> parts()
+	{
+		return frames;
+	}
+
+	//implements MultiPart
+	public void addPartLike(TileImage image, Part refPart)
+	{
+		addFrame(image, ((Frame)refPart).duration);
+	}
+
+	//implements MultiPart
+	public TileImage asTileImage()
+	{
+		return this;
+	}
+
 	public void addFrame(TileImage img, int duration)
 	{
 		totalDuration += duration;
@@ -98,7 +122,7 @@ public class Animation extends TileImage
 		}
 	}
 
-	public class Frame
+	public class Frame implements Part
 	{
 		public final TileImage frame;
 		public final int duration;
@@ -108,6 +132,11 @@ public class Animation extends TileImage
 			this.frame = frame;
 			this.duration = duration;
 		}
+
+		//implements TileImage.Part
+		public TileImage getImage() {
+			return frame;
+		}
 	}
 
 	private TileImage getDefaultImage()