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Add 'interlaced' to the thumbnail transformation parameters. (Do not add it to the wiki syntax which is already overcomplicated, just to the thumb.php parameters.) Primarily for JPEG; other bitmap formats as long as it's not much extra effort.

This is step 1 of https://phabricator.wikimedia.org/T120032.

You can look at thumb.php for how thumbnail parameters are handled, and work backwards from there. The interesting part is going to be in BitmapHandler. You'll also need to modify the VipsScaler extension which is used by Wikimedia (it's invoked via the BitmapHandlerTransform hook).