gtk4paintablesink: Add env var to fullscreen window

For testing purposes with e.g. gst-launch.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1555>

video/gtk4/src/sink/imp.rs

@@ -713,6 +713,9 @@ impl PaintableSink {
                 window.set_child(Some(&picture));
             }
             window.set_default_size(640, 480);
+            if std::env::var("GST_GTK4_WINDOW_FULLSCREEN").as_deref() == Ok("1") {
+                window.set_fullscreened(true);
+            }
 
             window.connect_close_request({
                 let self_ = self_.clone();