Issue Description:
After switching from jpeg to WebP, the WebP images get rendered and appear on the filesystem, but when rendering finishes, the WebP files get deleted, while the old jpeg files remain.
I originally assumed this was an issue with WebP specifically, which is not the case.
When switching from one file format to another, and then doing a fullrender, the tiles in the new format get generated properly and appear on the filesystem, but when rendering finishes, all new tiles get deleted.
I assume what fullrender is trying do here is to delete old tiles if there are any, but it instead deletes the new tiles. This behavior only affects the fullrender command, when doing a radius render, the new tiles stay.
I tested this by switching from JPEG to WebP, and, after removing the tiles, by switching from WebP to JPEG. I observed the above behavior both times.

• Dynmap Version: 3.1-beta5-spigot

• Server Version: Spigot git-Spigot-a19903d-167ff59 (MC: 1.16.4)

• Pastebin of Configuration.txt: https://pastebin.com/6ArKTxRv

• Server Host (if applicable): local

• Pastebin of crashlogs or other relevant logs:

• Other Relevant Data/Screenshots:

• Steps to Replicate:

  • Set up a fresh and clean Spigot with Dynmap, don't change Dynmap settings
  • Start the server, run dynmap fullrender world, wait till it finishes, stop the server
  • Change the image-format setting to webp-q100
  • Start the server, run dynmap fullrender world, and watch plugins/dynmap/web/tiles/world/t/-1_-1/.

  As a faster alternative, you can run dynamp fullrender world:flat and watch the respective directory.

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