image/zoom: Zoom >=1 no best fit for ExactVolatile

The best-fit calculation would consider levels >=1 for best-fit. This
seems fundamentally wrong since that should be reserved for LargeFit.

This caused zoom-in to fail since best-fit was considered instead. But
that best-fit was computed with BestFit mode since the mode changed in
between in zoom_to_full.

Fixes #556

src/widgets/image/zoom.rs

@@ -48,9 +48,9 @@ impl imp::LpImage {
 
         let max_zoom_factor = match self.fit_mode.get() {
             // Do not allow to zoom larger than original size
-            FitMode::BestFit => 1.,
+            FitMode::BestFit | FitMode::ExactVolatile => 1.,
             // Allow arbitrary zoom
-            FitMode::LargeFit | FitMode::ExactVolatile => f64::MAX,
+            FitMode::LargeFit => f64::MAX,
         };
 
         let default_zoom = if texture_aspect_ratio > widget_aspect_ratio {