Size a panorama's cell and thumbnail by class: two, three or four columns

One lookup, natural_span, maps a photograph's aspect to the columns its
cell spans, and the same number names its thumbnail class, Wide2, Wide3
or Wide4, 512 pixels of long edge per column, so a 4:1 panorama is as
sharp across four columns as a frame is in one. The boundaries are
where the two neighbouring cells would leave the same share of
themselves undrawn, sqrt(s(s+1)): 2.45 and 3.46, with the first at 1.9
so a 3:2 frame stays a frame. A grid too narrow for the class falls
back to the widest that fits, the tablet gives the whole row, and a
cell asks for the class it is actually drawn at: its span, but never
more than its own class. The merge renders each wide class up to the
composite's own, which covers every fallback.

ui/dr-ui/src/library_ui/window.rs

@@ -526,7 +526,7 @@ pub(super) fn load_window(window: &AppWindow, ctl: &Rc<LibraryController>) {
             let Some(made) = merged.get(&c.image_id) else {
                 continue;
             };
-            let wanted = dr_thumbs::ThumbSize::for_span(layout.span_of(offset + i), cell_pixels);
+            let wanted = dr_thumbs::ThumbSize::for_span(layout.class_span(offset + i), cell_pixels);
             if held
                 .get(&c.image_id)
                 .is_some_and(|h| h.class == Some(wanted))
@@ -755,7 +755,7 @@ fn request_thumbnails(window: &AppWindow, ctl: &Rc<LibraryController>) {
     let classes: Vec<dr_thumbs::ThumbSize> = {
         let layout = ctl.layout.borrow();
         (0..ctl.paths.borrow().len())
-            .map(|i| dr_thumbs::ThumbSize::for_span(layout.span_of(offset + i), cell_pixels))
+            .map(|i| dr_thumbs::ThumbSize::for_span(layout.class_span(offset + i), cell_pixels))
             .collect()
     };