Fix F6 shortcut to show the timeline thumbnail by default (fix asepri…

…te#4020)

Before this fix, when the thumbnail size was 1, the "Toggle Timeline
Thumbnails" command (F6 key) seemed to not work until we changed
the timeline thumbnail size to 2 or more.

src/app/ui/timeline/timeline.cpp

@@ -313,6 +313,9 @@ void Timeline::setZoomAndUpdate(const double zoom,
 
 void Timeline::onThumbnailsPrefChange()
 {
+  if (docPref().thumbnails.enabled() &&
+      docPref().thumbnails.zoom() == 1)
+    docPref().thumbnails.zoom(2);
   setZoomAndUpdate(
     docPref().thumbnails.enabled() ?
     docPref().thumbnails.zoom(): 1.0,