Reuse the face grid's decoded crops across a redraw

A confirm or a reject changes one row and redraws the whole grid, and the
redraw re-read every crop blob of the selected person (4 MB for the
largest) and decoded every one — 316 ms per click on the reference
library's 754-face person, to arrive at the pixels already on screen.

`load_faces` now takes the crops the previous load decoded, keyed by face,
and moves each into its new cell; the blob read is skipped when every face
is already in hand. `refresh` drains the old cells into it rather than
cloning them. The redraw is 2.6 ms.

ui/dr-ui/examples/identity_bench.rs

@@ -60,8 +60,17 @@ fn main() {
     time("load_people", 5, || {
         identity::load_people(&catalog, MODEL_ID).unwrap();
     });
-    time("load_faces (largest person)", 5, || {
-        identity::load_faces(&catalog, &store, biggest).unwrap();
+    time("load_faces (largest person, cold)", 5, || {
+        identity::load_faces(&catalog, &store, biggest, Default::default()).unwrap();
+    });
+    // What a confirm click costs: the grid's own crops handed back in.
+    let mut cells = identity::load_faces(&catalog, &store, biggest, Default::default()).unwrap();
+    time("load_faces (largest person, redraw)", 5, || {
+        let cut = cells
+            .drain(..)
+            .filter_map(|c| Some((c.face, c.crop?)))
+            .collect();
+        cells = identity::load_faces(&catalog, &store, biggest, cut).unwrap();
     });
     time("audit (coverage line)", 5, || {
         dr_ui::faces::audit(&catalog, &store, MODEL_ID, &registry).unwrap();