struct Gradient

A two-stop linear ramp, for .background(_:).

Two stops rather than an arbitrary list, and linear rather than radial, because that is what the renderer evaluates exactly: the ramp is computed per vertex and interpolated across the quad, and a two-stop linear gradient is the one family that survives that without error. Multi-stop and radial would need the ramp evaluated per fragment, which is a different pipeline rather than a longer array here.

Both stops carry their own alpha, so a fade to transparent is an ordinary gradient with to: .clear rather than a separate mechanism.