fix #1826 set splitSize to 3

src/main/java/org/codelibs/fess/thumbnail/ThumbnailManager.java

@@ -74,7 +74,7 @@ public class ThumbnailManager {
 
     protected String imageExtention = "png";
 
-    protected int splitSize = 2;
+    protected int splitSize = 3;
 
     protected int thumbnailTaskQueueSize = 10000;