gtk4: Declare correct default value for force-aspect-ratio property

It's defaulting to false as generally keeping the aspect ratio is the
job of the widget layout and not the paintable.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1654>

video/gtk4/src/sink/paintable/imp.rs

@@ -99,7 +99,7 @@ impl ObjectImpl for Paintable {
                 glib::ParamSpecBoolean::builder("force-aspect-ratio")
                     .nick("Force Aspect Ratio")
                     .blurb("When enabled, scaling will respect original aspect ratio")
-                    .default_value(true)
+                    .default_value(false)
                     .build(),
             ]
         });