feat: 🎸 revert loaded theme

[theashraf]

Changes:

• Updated ThorVG to the latest v0.12.x, which includes an enhancement to slots. This update enables the removal of applied slots by passing a nullptr to the override method.
• Updated the load_theme method in dotlottie_player to automatically revert to the default theme when an empty theme_id is passed.

let dotlottie_player = DotLottiePlayer::new(Config{...});

dotlottie_player.load_theme("dark_theme_id"); // This will load the "dark_theme_id" if it exists in the loaded .lottie asset

dotlottie_player.load_theme(""); // This will remove the previously applied theme and revert to the default animation

[changeset-bot]

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Latest commit: 5bdfee1

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