fix length of IDAT chunk

src/main/java/eu/jonahbauer/raytracing/render/ImageFormat.java

@@ -1,6 +1,7 @@
 package eu.jonahbauer.raytracing.render;
 
 import eu.jonahbauer.raytracing.render.canvas.Canvas;
+import eu.jonahbauer.raytracing.render.canvas.Image;
 import org.jetbrains.annotations.NotNull;
 
 import java.io.*;
@@ -10,6 +11,7 @@ import java.nio.file.Path;
 import java.util.zip.CRC32;
 import java.util.zip.CheckedOutputStream;
 import java.util.zip.DeflaterOutputStream;
+import java.util.zip.InflaterInputStream;
 
 public enum ImageFormat {
     PPM {
@@ -93,7 +95,7 @@ public enum ImageFormat {
                 }
 
                 var bytes = baos.toByteArray();
-                data.writeInt(bytes.length);
+                data.writeInt(bytes.length - 4); // don't include type in length
                 data.write(bytes);
                 data.writeInt((int) crc.getChecksum().getValue());
             }