Smooth scrim gradient

A scrim is the block of gradient that sits between artwork and the page below it, so the image appears to dissolve into the background. Written the obvious way — Brush.verticalGradient(listOf(Color.Transparent, background)) — it produces two artefacts at once: a visible line where the fade begins, and a dirty band through the middle. Both are fixable, and neither is fixed by adding more alpha.