feat(gif): Add GIF animation loop count detection for animated WebP conversion

[skidder]

Description

This PR adds functionality to detect and propagate loop count information from animated GIF sources to WebP outputs. Previously, all GIFs were treated as infinitely looping, but this change allows proper handling of GIFs with specific loop counts (e.g., play once, play N times).

Changes

• Added giflib_decoder_get_loop_count() C++ function to parse NETSCAPE2.0 extension blocks
• Extended Go GIF decoder struct to cache loop count information
• Updated GIF decoder's LoopCount() method to return actual loop count
• Added test case using a non-looping GIF file

Technical Details

• The NETSCAPE2.0 extension block is parsed to extract loop count information
• Default loop count is 1 (play once) if no extension block is found
• Loop count of 0 indicates infinite looping
• Loop count information is cached to avoid repeated parsing

Testing

• Added test case with a non-looping GIF file
• Verified loop count propagation through the WebP encoder pipeline
• Existing tests continue to pass