Fix Gtk3Backend image toggling bug

When images got shown after the first render, they were remaining invisible. For
some reason Gtk 3 wants us to explicitly show the widget after changing its
pixbuf (or changing its parent). This isn't true for any other widgets afaik.
The fix was to explicitly show the image widget at the end of updateImageView.

Author: stackotter

Sources/Gtk3Backend/Gtk3Backend.swift

@@ -676,6 +676,7 @@ public final class Gtk3Backend: AppBackend {
         )
 
         imageView.setCairoSurface(surface)
+        imageView.show()
     }
 
     // private class Tables {