perf: general minor performance optimisations

Notably there is some code cleanup with the zooming functionality, I've
created a new module to reduce code duplication.

src/zoom.rs

@@ -0,0 +1,52 @@
+use std::num::NonZeroU16;
+
+use crate::{config::IconSizes, tab::View};
+
+static DEFAULT_ZOOM: NonZeroU16 = NonZeroU16::new(100).unwrap();
+static MIN_ZOOM: NonZeroU16 = NonZeroU16::new(50).unwrap();
+static MAX_ZOOM: NonZeroU16 = NonZeroU16::new(500).unwrap();
+const ZOOM_STEP: u16 = 25;
+
+pub(crate) const fn zoom_to_default(view: View, icon_sizes: &mut IconSizes) {
+    let icon_size = select_resized_icon(view, icon_sizes);
+    *icon_size = DEFAULT_ZOOM;
+}
+
+pub(crate) fn zoom_in_view(view: View, icon_sizes: &mut IconSizes) {
+    let icon_size = select_resized_icon(view, icon_sizes);
+
+    let mut step = MIN_ZOOM;
+    while step <= MAX_ZOOM {
+        if *icon_size < step {
+            *icon_size = step;
+            break;
+        }
+        step = step.saturating_add(ZOOM_STEP);
+    }
+    if *icon_size > step {
+        *icon_size = step;
+    }
+}
+
+pub(crate) fn zoom_out_view(view: View, icon_sizes: &mut IconSizes) {
+    let icon_size = select_resized_icon(view, icon_sizes);
+
+    let mut step = MAX_ZOOM;
+    while step >= MIN_ZOOM {
+        if *icon_size > step {
+            *icon_size = step;
+            break;
+        }
+        step = NonZeroU16::new(step.get().saturating_sub(ZOOM_STEP)).unwrap();
+    }
+    if *icon_size < step {
+        *icon_size = step;
+    }
+}
+
+const fn select_resized_icon(view: View, icon_sizes: &mut IconSizes) -> &mut NonZeroU16 {
+    match view {
+        View::Grid => &mut icon_sizes.grid,
+        View::List => &mut icon_sizes.list,
+    }
+}