ZeroCopyEncoder::new now takes an output path and writes a real container
(format inferred from the extension, e.g. .mp4): create an output format
context, add the h264 stream from the encoder, write header; encode_rgba
rescales each packet's ts and av_interleaved_write_frame's it; finish
flushes + writes the trailer + closes. Sets AV_CODEC_FLAG_GLOBAL_HEADER for
mp4/mov so SPS/PPS land in extradata. This lets the editor's existing
mux_video_and_audio consume the temp video file unchanged.
The zerocopy_encode test now writes a .mp4 and ffprobe-verifies the codec,
dimensions, and frame count. Also let wgpu own the imported plane-image
destruction via texture_from_raw drop callbacks (clears two warnings).
a00e73c4b3