The main reason why I want to revert this is because the code doesn't actually work right now due to imagemagick not being installed. The code path is currently not being used, but I would like to start using it again (see https://github.com/dolphin-emu/dolphin/pull/9275).
This reverts commit 2a9794dd84575e637ae9f55b0c0ad4b355b24107.
The commit that made Dolphin stop outputting alpha was merged 5 years ago (https://github.com/dolphin-emu/dolphin/pull/3157), so there's no longer much reason to keep this around.
The main reason why I want to revert this is because the code doesn't actually work right now due to imagemagick not being installed. The code path is currently not being used, but I would like to start using it again (see https://github.com/dolphin-emu/dolphin/pull/9275).