Capture RCO image data (PNG) for the renderer

RCO IMG entries hold PNG images (magic 89 50 4E 47), decodable directly
by ImageIO/JavaFX. RcoFile now reads pImgData and stores raw image
bytes in ImageObject; the CLI reports the first image's magic.

cli/src/main/java/dev/quoije/umd/cli/Main.java

@@ -409,6 +409,14 @@ public final class Main {
             System.out.printf("%s: valid=%b compression=%d objects=%d images=%d%n",
                     rcoPath, rco.isValid(), rco.compression(), rco.objects().size(), rco.images().size());
 
+            for (Map.Entry<Integer, dev.quoije.umd.rco.object.ImageObject> img : rco.images().entrySet()) {
+                byte[] imgData = img.getValue().getData();
+                System.out.printf("  image 0x%X: %d bytes, magic=%s%n",
+                        img.getKey(), imgData == null ? 0 : imgData.length,
+                        imgData == null ? "null" : hexPrefix(imgData, 8));
+                break;
+            }
+
             System.out.printf("events (%d):%n", rco.events().size());
             for (Map.Entry<Integer, String> event : rco.events().entrySet()) {
                 System.out.printf("  [%3d] %s%n", event.getKey(), event.getValue());