screenshot-window-sizer: Handle wrapping around backwards

The mod takes care of wrapping at the top, but not when
decrementing 0. Fix this by using Array.prototype.at(),
which handles negative indexes.

Close https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/issues/467

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/284>
(cherry picked from commit dd8ee2931a)

extensions/screenshot-window-sizer/extension.js

@@ -96,7 +96,7 @@ export default class ScreenshotWindowSizerExtension extends Extension {
 
         // get the next size up or down from ideal
         let newIndex = (nearestIndex + (backwards ? -1 : 1)) % scaledSizes.length;
-        let [newWidth, newHeight] = scaledSizes[newIndex];
+        let [newWidth, newHeight] = scaledSizes.at(newIndex);
 
         // Push the window onscreen if it would be resized offscreen
         let newX = outerRect.x;