fix scale/rotate/rotateScale distortions + tests (publiclab#380)

* test scale

* no more distortions

* improve revert

* refactor

* update docs

* move comment out of code into README

* clean tests

* clean

* clean

* refactor

README.md

@@ -510,10 +510,35 @@ We have made slight changes to a default Leaflet handler:
   <ul><li>Returns the coordinates of the image corner at <i>index</i>.</li></ul>
 </details>
 
+<<<<<<< Upstream, based on Leaflet.DistortableImage/main
 <details><summary><code><b>getCorners()</b>: 4 [LatLng, LatLng, LatLng, LatLng]</code></summary>
   <ul><li>Returns the coordinates of the image corners in NW, NE, SW, SE order.</li></ul>
 </details>
+=======
+- `setCorner(idx, latLng)` - update an individual image corner and, where applicable, marker and toolbar positioning. We use this internally for `distort` mode.
+
+- `setCorners(corners)` - same as`setCorner`, but takes in a "corners" object to update all 4 corners with only one UI update at the end. We use this internally for image translation, rotation, and scaling. 
+   - Ex. 
+   ```JS
+   /** keys: 0-4, values: L.latLng objects for desired corner positions */
+    scaledCorners = {0: '', 1: '', 2: '', 3: ''};
+
+    for (i = 0; i < 4; i++) {
+      p = map
+        .project(this.getCorner(i))
+        .subtract(center)
+        .multiplyBy(scale)
+        .add(center);
+      scaledCorners[i] = map.unproject(p);
+    }
+
+    this.setCorners(scaledCorners);
+  ```
+
+- `getCenter()` - Calculates the centroid of the image.
+>>>>>>> 2d8a1db fix scale/rotate/rotateScale distortions + tests (#380)
 
+<<<<<<< Upstream, based on Leaflet.DistortableImage/main
 <details><summary><code><b>setCorner(<i>idx</i> &#60;number 0..3>, <i>LatLng</i>)</b>: this</code></summary>
   <ul>
     <li>Updates the coordinates of the image corner at <i>index</i> to <i>LatLng</i> and, where applicable, marker and toolbar positioning.</li>
@@ -622,6 +647,16 @@ A handler that holds the keybindings and toolbar API for an image instance. It i
 </details>
 
 ---
+=======
+- `scaleBy(num)` - scales the image by the given ratio and calls `setCorners`.
+   - ex. `overlay.scaleBy(0.5)`
+   - a scale of 0 or 1 will leave the image unchanged - but 0 causes the function to automatically return
+   - a negative scale will invert the image and, depending on the scale value, change its size
+
+- `rotateBy(rad)` - rotates the image by the given radian angle and calls `setCorners`.
+
+<hr>
+>>>>>>> 2d8a1db fix scale/rotate/rotateScale distortions + tests (#380)
 
 `L.DistortableCollection`