Add opacity to style and implement it as a renderer effect

[jrmoulton]

add the ability to add opacity as a style and then implement opacity in each of the renderers.

It should be tracked in the paint context and should work with save/restore so that entire elements can be given a new opacity.

In vello, this can be implemented by pushing a new layer with the opacity.
Tiny skia probably also has something built in to enable this.
Vger might be tricky.