Issue Description: Exception is thrown while rendering Zoom Out Tiles and Tiles not shown on Web

• Dynmap Version: 3.4-beta-2

• Server Version: git-Paper-232 (MC: 1.18.2)

• Pastebin of Configuration.txt: https://pastebin.com/4DJ7aSNd

• Server Host (if applicable): Selfhosted, Archlinux

• Other Relevant Data/Screenshots:
  [00:15:31 ERROR]: [dynmap] Exception during render job: org.dynmap.MapManager_DoZoomOutProcessing@754d2820
  [00:15:31 WARN]: java.lang.StringIndexOutOfBoundsException: begin 0, end 17, length 16
  [00:15:31 WARN]: at java.base/java.lang.String.checkBoundsBeginEnd(String.java:4601)
  [00:15:31 WARN]: at java.base/java.lang.String.substring(String.java:2704)
  [00:15:31 WARN]: at Dynmap-3.4-beta-2-spigot.jar//org.dynmap.storage.filetree.FileTreeMapStorage$StorageTile.(FileTreeMapStorage.java:47)
  [00:15:31 WARN]: at Dynmap-3.4-beta-2-spigot.jar//org.dynmap.storage.filetree.FileTreeMapStorage$StorageTile.getZoomOutTile(FileTreeMapStorage.java:219)
  [00:15:31 WARN]: at Dynmap-3.4-beta-2-spigot.jar//org.dynmap.DynmapWorld.processZoomFile(DynmapWorld.java:138)
  [00:15:31 WARN]: at Dynmap-3.4-beta-2-spigot.jar//org.dynmap.DynmapWorld.freshenZoomOutFiles(DynmapWorld.java:119)
  [00:15:31 WARN]: at Dynmap-3.4-beta-2-spigot.jar//org.dynmap.MapManager$DoZoomOutProcessing.run(MapManager.java:951)
  [00:15:31 WARN]: at Dynmap-3.4-beta-2-spigot.jar//org.dynmap.MapManager$DynmapScheduledThreadPoolExecutor$2.run(MapManager.java:252)
  [00:15:31 WARN]: at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:539)
  [00:15:31 WARN]: at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
  [00:15:31 WARN]: at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304)
  [00:15:31 WARN]: at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
  [00:15:31 WARN]: at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
  [00:15:31 WARN]: at java.base/java.lang.Thread.run(Thread.java:833)

• Steps to Replicate: don't know, appears on every start of Server and always when Zoom Out tiles are beeing processed.., even on absolute fresh Server, new fullrender, and completly different map...

[x] I have looked at all other issues and this is not a duplicate
[x] I have been able to replicate this

Current code has implementation limit on zoom levels (at 16), which is the root of this problem. The current SNAPSHOT build should address this (767 or later).

works fine, thank you