Synchronize usages of ImageIO.read(...)

ImageIO.read(...) is not really thread-safe, as calling it on several threads at the same time can cause NullPointerExceptions and ConcurrentModificationExceptions, as reported in https://bugs.openjdk.java.net/browse/JDK-8058973 and https://bugs.openjdk.java.net/browse/JDK-6986863.

This commit changes all usages of this method to synchronize on ImageIO.class, so the method is never run asynchronously.

http-api/src/main/java/net/runelite/http/api/item/ItemClient.java

@@ -95,8 +95,10 @@ public class ItemClient
 			}
 
 			InputStream in = response.body().byteStream();
-			BufferedImage imageIcon = ImageIO.read(in);
-			return imageIcon;
+			synchronized (ImageIO.class)
+			{
+				return ImageIO.read(in);
+			}
 		}
 	}