Improve AVIF animation frame handling

This PR improves the handling of animated AVIF files by updating the frame disposal and blending logic:

• For animated AVIFs, now uses DISPOSE_BACKGROUND by default to clear previous frames
• Only uses DISPOSE_NONE for frames with premultiplied alpha
• Uses BLEND_ALPHA when either premultiplied alpha or alpha plane is present
• Maintains backward compatibility for non-animated images

These changes ensure proper handling of transparency in animated AVIF sequences, particularly for files with non-premultiplied alpha channels.

Specification Alignment

The changes align with the AVIF specification (ISO/IEC 23008-12):

Frame Disposal

• DISPOSE_BACKGROUND for non-premultiplied alpha to ensure proper transparency
• DISPOSE_NONE for premultiplied alpha frames designed for direct compositing

Blending Behavior

• BLEND_ALPHA when alpha is present (premultiplied or not)
• BLEND_NONE for frames without transparency