Fix GIF decode failure on streams that fill the LZW code table - #141
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GIF89a caps LZW codes at 12 bits, so the code table stops at 4096 entries. compress_lzw has always honoured that -- it emits a Clear Code rather than adding entry 4096 -- but decompress_lzw widened straight off its own counter with no ceiling. The decoder trails the encoder by exactly one table entry, so it reaches counter == 4096 while reading the last code the encoder wrote before that Clear. get_bits_number_for(4096) is 13, so the next read took 13 bits out of a stream still written in 12. Every code after that was shifted: a Clear Code was mis-detected, the table reset, and the reader then asked for a code past the end of it. In the editor that faulted with "Nonexistent function add in base Nil" at lzw.gd:193. In a release build decompress_lzw simply abandoned the frame, so load_gif returned a SpriteFrames whose get_frame_count() looked healthy while the textures were empty -- failing much later and much further away from the cause. Twitch first-party emotes are small enough never to approach the boundary, which is why this went unnoticed; the reported trigger was a 318-frame 7TV emote that genuinely uses the full 12-bit space. Adds a generated GIF fixture that reproduces it end to end. Without this fix, test_round_trips_a_stream_that_fills_the_code_table and test_decodes_a_gif_that_fills_the_lzw_code_table both fail. Fixes #132 Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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closes: #132