Fix error thrown in MC Versions <1.17

* fix invisible/fixed toggle not working on glowing item frames

• added support for glowing Item frames

Thanks to @DerEnderKeks for providing this patch!

* added User Agent, to increase compatibility with CDNs
* added delete command
* offloaded image color conversion to async task, improving performance
* fixed bug that could cause artifacts when scaling images

Thanks to DerEnderKeks for providing the first two additions.

Any file could be downloaded as long as the webserver claimed they were images. This can allow a compromised or malicious server to serve any kind of data to the requesting server, including executable code.

The risk for this being exploited is very minimal, the downloaded files can't be executed or used for anything malicious without either another exploit or additional actions by a malicious, compromised or non-suspecting user.

Nevertheless, this update adds an additional verification layer making sure the downloaded file is a valid image file.

As a general rule of thumb you should

• always use a secure connection to download files (https)
• only give trusted and properly secured users access to the download functionality
• only download from trusted sites
• preferably upload the images manually