skin overrides

• textures are no longer eagerly loaded, which should reduce memory usage when using iris (gh#21)
• valid PNGs are now always accepted, even if they aren't named .png
• uploading a file which is not a PNG now gives a warning instead of crashing (gh#20)
• fixed ears sometimes not working on 1.21.4+
  • the builtin SkinTextureDownloader.registerTextureInManager method is now used, allowing it to be used for injecting
• fixed another mystery crash (gh#19)

fixed an crash caused by mods misusing player skulls on 1.15.2 - 1.20.1.