Make rotation work without a focus point

Close #140

crates/fj-viewer/src/input/rotation.rs

@@ -1,34 +1,40 @@
-use nalgebra::{Rotation3, Translation, Vector};
+use nalgebra::{Point, Rotation3, Translation, Vector};
 
 use crate::camera::{Camera, FocusPoint};
 
 pub struct Rotation {
+    active: bool,
     focus_point: FocusPoint,
 }
 
 impl Rotation {
     pub fn new() -> Self {
         Self {
+            active: false,
             focus_point: FocusPoint::none(),
         }
     }
 
     pub fn start(&mut self, focus_point: FocusPoint) {
+        self.active = true;
         self.focus_point = focus_point;
     }
 
     pub fn stop(&mut self) {
-        self.focus_point = FocusPoint::none();
+        self.active = false;
     }
 
     pub fn apply(&self, diff_x: f64, diff_y: f64, camera: &mut Camera) {
-        if let Some(focus_point) = self.focus_point.0 {
+        if self.active {
+            let rotate_around =
+                self.focus_point.0.unwrap_or_else(Point::origin);
+
             let f = 0.005;
 
             let angle_x = diff_y * f;
             let angle_y = diff_x * f;
 
-            let trans = Translation::from(focus_point.coords);
+            let trans = Translation::from(rotate_around);
 
             let rot_x = Rotation3::from_axis_angle(&Vector::x_axis(), angle_x);
             let rot_y = Rotation3::from_axis_angle(&Vector::y_axis(), angle_y);