Major ImageMetadata refactor for more consistent standard metadata support.

Fixes a few related bugs as a bonus.

imageio/imageio-pict/src/test/java/com/twelvemonkeys/imageio/plugins/pntg/PNTGMetadataTest.java

@@ -1,7 +1,11 @@
 package com.twelvemonkeys.imageio.plugins.pntg;
 
+import com.twelvemonkeys.imageio.util.ImageTypeSpecifiers;
+
 import org.junit.Test;
 
+import java.awt.image.*;
+
 /**
  * PNTGMetadataTest.
  *
@@ -12,6 +16,6 @@ import org.junit.Test;
 public class PNTGMetadataTest {
     @Test
     public void testCreate() {
-        new PNTGMetadata();
+        new PNTGMetadata(ImageTypeSpecifiers.createFromBufferedImageType(BufferedImage.TYPE_BYTE_BINARY));
     }
 }