Introduce a dynamic coefficient for touchmove yaw/pitch change speed

With a fixed coefficient affecting touchmove pan speed, at large
values of config.hfov (zoomed out) panning will feel "sluggish"
due to panorama panning less than finger is moved on screen.
On the other hand, at small config.hfov values (zoomed in),
panning will be perceived to move faster.

To improve usability at both small and large config.hfov
(zoomed-out and zoomed-in) we introduce a dynamic coefficient
affected by the config.hfov value.

Currently this seems to roughly keep initial drag/pan start position
close to the user's finger while panning regardless of zoom level.

src/js/pannellum.js

@@ -736,12 +736,21 @@ function onDocumentTouchMove(event) {
             setHfov(config.hfov + (onPointerDownPointerDist - clientDist) * 0.1);
             onPointerDownPointerDist = clientDist;
         }
-        
-        var yaw = (onPointerDownPointerX - clientX) * 0.1 + onPointerDownYaw;
+
+        // The smaller the config.hfov value (the more zoomed-in the user is), the faster
+        // yaw/pitch are perceived to change on one-finger touchmove (panning) events and vice versa.
+        // To improve usability at both small and large zoom levels (config.hfov values)
+        // we introduce a dynamic pan speed coefficient.
+        //
+        // Currently this seems to *roughly* keep initial drag/pan start position close to
+        // the user's finger while panning regardless of zoom level / config.hfov value.
+        var touchmovePanSpeedCoeff = config.hfov / 360;
+
+        var yaw = (onPointerDownPointerX - clientX) * touchmovePanSpeedCoeff + onPointerDownYaw;
         yawSpeed = (yaw - config.yaw) % 360 * 0.2;
         config.yaw = yaw;
-        
-        var pitch = (clientY - onPointerDownPointerY) * 0.1 + onPointerDownPitch;
+
+        var pitch = (clientY - onPointerDownPointerY) * touchmovePanSpeedCoeff + onPointerDownPitch;
         pitchSpeed = (pitch - config.pitch) * 0.2;
         config.pitch = pitch;
     }