[FFmpeg-devel] [fate] add webp/dual_transform.webp

[James Zern]

Reviving this from samples-request at ffmpeg.org. The patch has been
merged, all that is needed is to add the file and enable the test.

The file can be downloaded from gitiles without needing to clone the
repo [1]. This is to add coverage for a fix for 9368 [2][3].

[1] curl "https://chromium.googlesource.com/webm/libwebp-test-data/+/refs/heads/main/dual_transform.webp?format=TEXT"
|base64 -d >dual_transform.webp
[2] https://trac.ffmpeg.org/ticket/9368
[3] https://patchwork.ffmpeg.org/project/ffmpeg/patch/20210830121051.1703262-1-maryla@google.com/