fix: prevent mixed formats per source in Picture generation

[tijmenbruggeman]

The 'smallest' compression can be different for various sizes, sometimes in webp and higher resolution images would be in AVIF:

If AVIF is not supported, it would only show lowest resolution images to visitors. To prevent this, we will no longer have mixed formats in the sources. To prevent this from happening with existing convertions, we will only select a source if the largest resolution is part of the srcset.

Changes

• Modified the signature to compress_file to only send the mimetypes to which it can convert to ($convert_to)
• This $convert_to parameter is initially set by checking the settings. If the original image is already compressed, then it will use the converted mimetype of the original image.
• parse width description to check if alternative sources have the largest width in their set.