Merge page: outline every frame on the preview, and let it be tall

A sweep whose frames overlap by more than half reads as one photograph,
and the page's job is to show frames. Each footprint is walked along its
border and drawn in amber where it lands, so twelve frames look like
twelve and a misplaced one is visible as such. The preview may take most
of the page's height rather than 320 px.

ui/dr-ui/src/merge.rs

@@ -603,6 +603,41 @@ fn preview(
             out[o + 3] = 255;
         }
     }
+
+    // Each frame's outline, so the page shows *frames* and not one picture:
+    // a sweep of frames overlapping by more than half reads as a single
+    // photograph otherwise, which is what a photographer looking for a
+    // misplaced frame needs to see through. Walked along each border and
+    // drawn where it lands, two pixels wide, in a colour the scene does not
+    // have.
+    let steps = 600;
+    for k in 0..proxies.len() {
+        for s in 0..steps {
+            let t = s as f64 / steps as f64;
+            for p in [
+                (-fw / 2.0 + fw * t, -fh / 2.0),
+                (-fw / 2.0 + fw * t, fh / 2.0 - 1.0),
+                (-fw / 2.0, -fh / 2.0 + fh * t),
+                (fw / 2.0 - 1.0, -fh / 2.0 + fh * t),
+            ] {
+                let d = cameras.bearing(k, p);
+                let Some((u, v)) = projection.from_direction(scale, d) else { continue };
+                let x = ((u - bounds.min_u) / px) as i64;
+                let y = ((v - bounds.min_v) / px) as i64;
+                for (dx, dy) in [(0, 0), (1, 0), (0, 1), (1, 1)] {
+                    let (xx, yy) = (x + dx, y + dy);
+                    if xx < 0 || yy < 0 || xx >= out_w as i64 || yy >= out_h as i64 {
+                        continue;
+                    }
+                    let o = (yy as usize * out_w + xx as usize) * 4;
+                    out[o] = 255;
+                    out[o + 1] = 176;
+                    out[o + 2] = 0;
+                    out[o + 3] = 255;
+                }
+            }
+        }
+    }
     (out_w as u32, out_h as u32, out)
 }