Fix #995: a width-profile stroke keeps its shape when the view zooms out

The canvas keeps a hairline visible by raising a stroke's weight to half a device pixel. A width profile multiplies that weight, so a 1 pt stroke with a 6x width point swelled to twice its size at 25% zoom and shrank back on zooming in. The minimum now goes to the profile's widest point instead; plain strokes are unchanged.

crates/render/src/tests.rs

@@ -29,6 +29,7 @@ mod setup;
 mod strokegradient;
 mod tileedge;
 mod viewraster;
+mod widthzoom;
 
 fn rect_node(d: &mut Document, r: Rect, fill: Paint, stroke: Paint, w: f64) -> Node {
     let id = d.alloc_id();