Image Trace: Shapes, and corners whose pixel is mostly outside the shape

Shapes (params `shapes`, a checkbox in the panel; off unless asked for): a filled
outline that is a circle, an ellipse, a rectangle or a regular polygon comes out
as that live shape. A shape is only made when every point of the traced outline
lies within 0.5 px (0.2% of the shape's size, if more) of it; an ellipse whose
radii differ by less than twice that is a circle of its area.

The outline before fitting: a corner's own pixel no longer pulls the two edges
that meet there, and a one-pixel step between two long edges is the corner where
they meet. Sharp corners kept: Default 69% -> 76%, the colour presets 86% -> 91%.

The harness gains plain-shape inputs and "shapes %".

crates/trace/src/logo/pipeline.rs

@@ -478,7 +478,13 @@ pub(crate) fn trace_logo(img: &Raster, params: &TraceParams, max_anchors: usize)
         if !out.palette.contains(&color) {
             out.palette.push(color);
         }
-        out.paths.push(TracedPath { path: PathData::new(subs), color, pixels: (s.area * (factor * factor) as f64) as usize, stroke: None });
+        out.paths.push(TracedPath {
+            path: PathData::new(subs),
+            color,
+            pixels: (s.area * (factor * factor) as f64) as usize,
+            stroke: None,
+            shape: None,
+        });
     }
     Ok(out)
 }