image: Settle with either rotate or zoom gesture

Add thresholds after which the gesture is assumed to be
only zoom or rotate.

Closes #95, #29

Part-of: <https://gitlab.gnome.org/Incubator/loupe/-/merge_requests/145>

src/util.rs

@@ -166,3 +166,10 @@ pub enum Direction {
     Back,
     Forward,
 }
+
+#[derive(Copy, Clone, Debug)]
+pub enum Gesture {
+    /// Rotate with threshold offset
+    Rotate(f64),
+    Scale,
+}