Draw black background from CSS when GL fails

When GLArea gets a GL error it leaves user with nothing, but transparent background with error text in the middle. Make it look somewhat decent by drawing black background for the not working video widget.

src/player.js

@@ -467,6 +467,16 @@ class ClapperPlayer extends PlayerBase
         this.widget.disconnect(this._realizeSignal);
         this._realizeSignal = null;
 
+        if(this.widget.get_error) {
+            const error = this.widget.get_error();
+            if(error) {
+                debug('player widget error detected');
+                debug(error);
+
+                this.widget.add_css_class('blackbackground');
+            }
+        }
+
         const root = this.widget.get_root();
         if(!root) return;