As first mentioned in commit 672171093, the CSS spec defines shadow
colors independently from the colors casting the shadow. It's not
a physical light simulation so a shadow is allowed to be a different
color from texture casting it.
This means we only care about the shape of the source where alpha
values of zero are adjacent to alpha values of non-zero. And all such
non-zero pixels should be treated as fully opaque for the purpose of
shadow generation. While this would be wrong for a physical light
simulation it does allow us to cast shadows around semi-translucent
shapes and better support CSS.
Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4477
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1918>
918c063693