Radial vignette darkening operation. :strength is in
[0.0, 1.0] and matches Image.vignette/2's argument of
the same name. Maps to Cloudinary's e_vignette[:N]
(N / 100, default 0.5).
Summary
Types
@type t() :: %Image.Plug.Pipeline.Ops.Vignette{strength: float()}