Paint source refactor

Previously, DrawingCtx.set_source_paint_server required a bunch of stuff: acquired_nodes, paint_server, opacity, bbox, current_color, values.

It's easier if the caller resolves the paint source and converts it to user space coordinates, and calls just set_paint_source(paint_source, acquired_nodes). This is done in several steps:

• A final RGBA color depends on a cssparser::Color, an opacity from stroke-opacity or fill-opacity, and the currentColor to resolve that case. Do that in a single resolve_color function.

• Gradients, instead of storing the plain stop-color values from the SVG code, now store the combined stop-color + stop_opacity + { stroke_opacity / fill_opacity }.

• Resolved patterns keep the { stroke_opacity / fill_opacity } around so it can be used when painting the generated pattern surface.

Also, I've started refactoring the filters code that deals with paint sources: filter primitives may take inputs that are FillPaint or StrokePaint. I'll stop refactoring right there to rebase !387 (closed); that looks like a good next step.

This MR started as an experiment in replacing DrawingCtx.with_saved_cr with a SavedCr struct that saves the cr when created, and restores it on Drop. This removes two nested function frames; I'm not yet sure if it is obvious that SavedCr really defines a context that lives for as long as its surrounding scope. It may let us remove the with_foo functions gradually.