gif: only the first frame renders, the rest silently draw nothing

[LeadcodeDev]

Symptom

A gif component shows its first frame and then nothing. No error, no warning — the scenario validates and the render succeeds.

Reproduction

ffmpeg -f lavfi -i "testsrc2=size=64x64:rate=12:duration=2" anim.gif   # 24 frames
# scenario: { "type": "gif", "src": "<abs>/anim.gif", "style": {"width":128,"height":128} }
rustmotion render -f gif.json -o f.png --frame 0    # 16140 non-black pixels
rustmotion render -f gif.json -o f.png --frame 5    # 0 non-black pixels

Cause

crates/rustmotion-components/src/gif.rs stores every frame as (frame.buffer, gif_width, gif_height), where gif_width/height come from decoder.width()/height() — the logical canvas. But an optimised GIF encodes each frame after the first as a sub-rectangle (frame.left/top/width/height), so frame.buffer is smaller than gif_width * gif_height * 4.

At paint time the ImageInfo declares the canvas size with row_bytes = gif_width * 4, so skia_safe::images::raster_from_data gets a buffer shorter than it needs, returns None, and the if let Some(img) arm is simply skipped — nothing is drawn and nothing is said. Frame 0 is full-canvas in most encoders, which is why it is the only one that appears.

Proposed fix

1. Compose frames instead of storing them raw. Keep one full-canvas RGBA buffer; for each decoded frame, blit frame.buffer at (frame.left, frame.top) over frame.width × frame.height, then snapshot it. This also fixes correctness beyond the crash: even at the right size, an optimised frame only carries the pixels that changed, so drawing it alone would show a fragment on an empty background.
2. Honour frame.dispose (Keep / Background / Previous) when composing, otherwise frames leak into each other.
3. Say something when a decode fails. The three silent exits (File::open, read_info, and the dropped raster_from_data) should surface the way fix(studio): re-analyse audio on every scenario load, and report failures #181 made the audio analysis surface its failures, rather than return.

Test

waveform and audio_spectrum have pixel-level tests; gif has none. A test rendering a two-frame GIF whose second frame is a sub-rectangle, asserting both frames differ and neither is empty, would have caught this.