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/ui/library.slint

@@ -486,7 +486,7 @@ export struct LibraryCell {
     // TRACES: FR-MRG-6
     // Where the cell sits in the grid's lattice of slots — row
     // `slot / columns`, column `slot % columns` — and how many slots it
-    // takes: one, or two or three for a panorama, or the whole row. Rust's
+    // takes: one, or two, three or four for a panorama, or the whole row. Rust's
     // layout (`library_ui::layout`) is the one place these are computed.
     slot: int,
     span: int,