Pick with the mouse

Click a tile to pick it, scroll to move the selection. Hovering does not
move the selection: the keyboard keeps that, and a click acts on whatever
is under the cursor instead. A click on the margin, on a gap, or on an
empty cell of a ragged last row does nothing.

Tiles are already subsurfaces, so a click on a thumbnail identifies its
tile by which surface the event arrived on — no hit-testing needed. Only
the chrome around them (padding, labels, gaps) needs Layout::hit, which
inverts the same maths elem() lays out with, and is tested against it.

Picking waits for press and release on the same tile, so sliding off a
tile before letting go is not a pick.

wp_cursor_shape_v1 sets the pointer shape, which spares us a cursor theme
and libwayland-cursor; without it the cursor keeps whatever shape the
window underneath gave it. It is optional — a compositor without it just
gets whatever shape was already there.

Cargo.toml

@@ -7,7 +7,7 @@ license = "MIT"
 
 [dependencies]
 wayland-client = "0.31"
-wayland-protocols = { version = "0.32", features = ["client", "staging"] }
+wayland-protocols = { version = "0.32", features = ["client", "staging", "unstable"] }
 wayland-protocols-wlr = { version = "0.3", features = ["client"] }
 memmap2 = "0.9"
 rustix = { version = "1", features = ["fs", "mm", "shm"] }