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/src/identity_ui.rs

@@ -260,7 +260,17 @@ pub fn refresh(
 
     match (ctl.selected.get(), store.as_deref()) {
         (Some(person), Some(store)) => {
-            let cells = identity::load_faces(cat, store, person).unwrap_or_else(|e| {
+            // The crops the grid is showing now, handed over to be reused
+            // rather than decoded again — see `load_faces`. Drained, not
+            // cloned: a crop is 64 KB of pixels and the largest groups hold
+            // hundreds.
+            let cut = ctl
+                .faces
+                .borrow_mut()
+                .drain(..)
+                .filter_map(|c| Some((c.face, c.crop?)))
+                .collect();
+            let cells = identity::load_faces(cat, store, person, cut).unwrap_or_else(|e| {
                 log::warn!("identity: reading faces: {e}");
                 Vec::new()
             });