Make ImageTracker.resolution readonly

docs/api/image-tracker.md

 
 ### resolution
 
-`tracker.resolution: Resolution`
+`tracker.resolution: Resolution, read-only`
 
 The [resolution](resolution.md) adopted by the computer vision algorithms implemented in the tracker. Higher resolutions improve the tracking quality, but are computationally more expensive. Note that this resolution is different from, and should not be larger than, the [resolution of the camera](camera-source.md#resolution)!
 
+*Note:* this property is read-only since 0.4.0. Set the resolution when instantiating the tracker.
+
 ## Events
 
 An ImageTracker is an [AREventTarget](ar-event-target.md). You can listen to the following events:

src/trackers/image-tracker/image-tracker.ts

     }
 
     /**
-     * Resolution of the AR screen space
+     * Resolution of the tracker
      */
     get resolution(): Resolution
     {
     }
 
     /**
-     * Resolution of the AR screen space
+     * Resolution of the tracker
+     * @readonly
      */
     set resolution(resolution: Resolution)
     {
-        this._resolution = resolution;
+        // this property is readonly, but this setter has been kept because
+        // it wasn't readonly in previous versions of the engine. FIXME
     }
 
     /**