image/zoom: Allow 50% zoom via menu for 16px images

Zoom out for 16px or smaller images is usually blocked. But allow it for
zoom menu as we allow 50% via zoom menu in other situations where it is
blocked otherwise. Also, this is probably much less complex
implementation wise.

Closes #544

src/widgets/image.rs

@@ -130,6 +130,9 @@ const SMALL_OCCUPY_SCREEN: f64 = 0.8;
 /// this value, unless the image itself is smaller than this.
 const SMALLEST_RENDERED_PIXEL_SIZE: f64 = 16.0;
 
+/// Smallest zoom level reachable via entries in the zoom menu
+const LOWEST_MENU_ZOOM_LEVEL: f64 = 0.5;
+
 #[derive(Default, Debug, Clone, Copy, glib::Variant, glib::Enum, PartialEq, Eq)]
 #[enum_type(name = "LpFitMode")]
 pub enum FitMode {