[feature request] support mpris caching for data uris

[danielwerg]

Describe the bug

mpris lib fails when fed base64 art_url

To Reproduce

1. Install https://github.com/hoyon/mpv-mpris or https://github.com/eNV25/mpv-mpris2
2. Play track that has art_url with mpv
3. See

player.vala:539: Failed to cache cover art with url "data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAgGBgcGBQgHBwcJCQgKDBQNDAsLDBkSEw8UHRofHh0aHBwgJC4nICIsIxwcKDcpLDAxNDQ0Hyc5PTgyPC4zNDL/2wBDAQkJCQwLDBgNDRgyIRwhMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjL/wAARCAAXAAsDASIAAhEBAxEB/8QAFgABAQEAAAAAAAAAAAAAAAAAAAMG/8QAFhABAQEAAAAAAAAAAAAAAAAAABIR/8QAFQEBAQAAAAAAAAAAAAAAAAAAAgb/xAAVEQEBAAAAAAAAAAAAAAAAAAAAEf/aAAwDAQACEQMRAD8Az1FJUUqqmIlpoAT/2Q==": The specified location is not supported

For testing purposes I was using data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAgGBgcGBQgHBwcJCQgKDBQNDAsLDBkSEw8UHRofHh0aHBwgJC4nICIsIxwcKDcpLDAxNDQ0Hyc5PTgyPC4zNDL/2wBDAQkJCQwLDBgNDRgyIRwhMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjIyMjL/wAARCAAXAAsDASIAAhEBAxEB/8QAFgABAQEAAAAAAAAAAAAAAAAAAAMG/8QAFhABAQEAAAAAAAAAAAAAAAAAABIR/8QAFQEBAQAAAAAAAAAAAAAAAAAAAgb/xAAVEQEBAAAAAAAAAAAAAAAAAAAAEf/aAAwDAQACEQMRAD8Az1FJUUqqmIlpoAT/2Q==

Expected behavior

Cache base64 or let user handle it

const coverArt = bind(player, 'coverArt').as(coverArt => {
  return coverArt ? coverArt : `${SRC}/fallback`; // won't work
});

Additional context

https://github.com/Aylur/astal/blob/main/lib/mpris/player.vala#L539

hoyon/mpv-mpris#111 eNV25/mpv-mpris2#72

[Aylur]

Currently it only supports uri schemes that Gio.File supports which is only file and http I think