Add Vello support for non-scaling stroke width

[Keavon]

See Stroke::non_scaling in style.rs.

This is used by the Outline view mode and works in our SVG renderer but isn't supported in our Vello renderer.

As for whether we allow users to set this on their own vector data, this needs some consideration because it violates our usual rule on scale invariance that's needed for consistent views of procedurally generated artwork regardless of scale. We could possibly make this the concept of a "hairline stroke" and find a way to draw it that's consistent across scales, such as making its thickness zero as far as any procedural operator nodes are concerned. Another option is to consider the "zoom factor" input to the Context call argument which lets nodes explicitly generate different content (such as level of detail in an OpenStreetMap view) when zooming (unless overridden and locked to a specific value in the navigation zoom popover).