Zooming out too far causes a panic in debug builds

[emilk]

❯ RUST_BACKTRACE=1 cargo run -p demo_native
…
thread 'main' panicked at walkers/src/tiles.rs:161:20:
attempt to subtract with overflow
stack backtrace:
   0: rust_begin_unwind
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/std/src/panicking.rs:665:5
   1: core::panicking::panic_fmt
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/panicking.rs:74:14
   2: core::panicking::panic_const::panic_const_sub_overflow
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/panicking.rs:181:21
   3: walkers::tiles::HttpTiles::placeholder_with_different_zoom
             at ./walkers/src/tiles.rs:161:20
   4: <walkers::tiles::HttpTiles as walkers::tiles::Tiles>::at::{{closure}}
             at ./walkers/src/tiles.rs:208:21
   5: core::option::Option<T>::or_else
             at /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/option.rs:1550:21
   6: <walkers::tiles::HttpTiles as walkers::tiles::Tiles>::at
             at ./walkers/src/tiles.rs:198:9
   7: walkers::map::flood_fill_tiles
             at ./walkers/src/map.rs:351:24
   8: <walkers::map::Map as egui::widgets::Widget>::ui
             at ./walkers/src/map.rs:220:13