I opened smolkling.webp in Photoshop, added a layer mask with a vertical
gradient, replaced the leftmost column with completely transparent
pixels (because the leftmost column is vertically predicted with the
horizontal filter too), and saved it as webp. That wasn't enough to
get a horizontal filter for the ALPH chunk though, so I also ran
cwebp \
-alpha_filter best \
smolkling.webp \
-o Tests/LibGfx/test-inputs/smolkling-vertical-alpha.webp
That did the trick.
a77b6c470a
