Loading animation still has broken transparency in Safari

[jpenilla]

especially noticeable in dark mode

not sure how I thought it was fixed in 713ac22

[jpenilla]

Now all 3 work in Safari on macOS 26.1 (before only one did), but still seems broken in Safari on iOS 26.1

In summary:

• Chrome/FF support webp transparency and will use that
• Safari macOS does not support webp transparency but does support this odd H.265+alpha format
• Safari iOS doesn't seem to support either of these, maybe there is a third format we can add? Or maybe my encoding to ProRes was wrong? (I used the same options as the article linked in the earlier commit for Shutter, which said it works on iOS as well)

I've submitted feedback to Apple along with a minimal reproducer, so maybe something will happen there with some luck.