player: register external plugins only once

In case of multiple graph tab the player was registering
several time the gtk4paintablesink

src/common.rs

@@ -12,6 +12,10 @@ use gtk::glib;
 pub fn init() -> Result<()> {
     std::env::set_var("GST_XINITTHREADS", "1");
     gst::init()?;
+    #[cfg(feature = "gtk4-plugin")]
+    {
+        gstgtk4::plugin_register_static().expect("Failed to register gstgtk4 plugin");
+    }
     gtk::init()?;
     Ok(())
 }

src/gps/player.rs

@@ -77,10 +77,6 @@ impl Player {
             current_state: Cell::new(PipelineState::Stopped),
             n_video_sink: Cell::new(0),
         }));
-        #[cfg(feature = "gtk4-plugin")]
-        {
-            gstgtk4::plugin_register_static().expect("Failed to register gstgtk4 plugin");
-        }
 
         Ok(pipeline)
     }