a11y: magnifier: change zoom stepping to multiplicative, bound max, add presets

1. Change magnifier zoom stepping from additive to multiplicative,
   because zoom is a ratio, not a difference, and human perception is
   logarithmic (browser zoom steps, map zoom levels, microscope powers).

2. Add `pub const ZoomConfig::ZOOM_INCREMENT_PRESETS` with two
   additional presets: 10% (macOS, Cinnamon) and 75% (for users at
   70-90x zoom).

3. Bound the zoom level to a generous maximum, beyond what anyone is expected
   to need, to prevent runaway / unrecoverable levels (#1230).

Full background and rationale at #2615.

I used AI to assist with research and issue writeup. Code is 100% mine.

- Closes #2615.
- Fixes #1239.
- Partially addresses #1230 and #1237.

src/input/actions.rs

@@ -29,6 +29,8 @@ use std::{os::unix::process::CommandExt, thread};
 
 use super::gestures;
 
+const MAX_ZOOM: f64 = 256.0;
+
 fn propagate_by_default(action: &shortcuts::Action) -> bool {
     matches!(
         action,
@@ -1116,7 +1118,13 @@ impl State {
         }
 
         if zoom_seat == *seat {
-            let new_level = (current_level + change).max(1.0);
+            let factor = 1.0 + change.abs();
+            let new_level = if change < 0. {
+                current_level / factor
+            } else {
+                current_level * factor
+            }
+            .clamp(1.0, MAX_ZOOM);
             shell.trigger_zoom(
                 seat,
                 Some(&output),