fix: Report frame-count, dispose-method, and background-color from GIF for use with animated WebP

[skidder]

This PR enhances the GIF decoder to properly handle animated GIF to WebP conversion, focusing on frame disposal methods and animation metadata.

Key Improvements

1. Proper Frame Disposal

   • Maps GIF disposal methods to WebP equivalents
   • Fixes visual artifacts in animations with transparent regions
   • Properly handles frame-to-frame transitions

2. Animation Metadata

   • Combined loop count and frame count reading into single pass
   • Cached animation info to prevent redundant processing
   • More accurate frame counting

3. Performance

   • Single-pass metadata reading
   • Cached animation information
   • Reduced file parsing overhead

These changes ensure that animated GIFs are correctly converted to WebP format while maintaining visual fidelity and animation timing.