workspace-indicator: Account for monitor offset in window previews

Windows' frame rects are in screen coordinates, while the workspace
thumbnails are based on the monitor work area. Unless we account
for the difference, previews end up mispositioned in multi-monitor
setups.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/142>

(cherry picked from commit 6949a5d075)

extensions/workspace-indicator/extension.js

@@ -97,8 +97,8 @@ class WorkspaceLayout extends Clutter.LayoutManager {
                 Math.round(Math.min(frameRect.width, workArea.width) * hscale),
                 Math.round(Math.min(frameRect.height, workArea.height) * vscale));
             childBox.set_origin(
-                Math.round(frameRect.x * hscale),
-                Math.round(frameRect.y * vscale));
+                Math.round((frameRect.x - workArea.x) * hscale),
+                Math.round((frameRect.y - workArea.y) * vscale));
             child.allocate(childBox);
         }
     }