fix: 🐛 ensure canvas is cleared before loading a new animation

[theashraf]

Fixes:

• Ensure tvg_canvas_clear(true) is called before loading a new animation to free the pushed paints from memory. This prevents memory leaks when loading multiple animations.
• Modify the thorvg::Animation struct to own the raw pointer to the picture, resulting in all related picture methods being moved to the Animation struct. This also simplifies the API.
• The width and height of the background_color shape should match the canvas's width and height, not the picture's width and height.
• Resize the background_color shape on canvas resize.

[changeset-bot]

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Latest commit: 0284121

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