Add paused-state vignette border to game world view

src/ui/GameWorldView.h

@@ -131,6 +131,10 @@ private:
     void drawBeams(QPainter& painter);
     void drawOverlays(QPainter& painter);
     void drawScreenSpace(QPainter& painter);
+    // Vignette-style border shown while the game is paused (speed 0x) to make the
+    // paused state hard to miss: a black frame whose alpha fades from 50% at the
+    // viewport edges to 0% toward the center.
+    void drawPauseBorder(QPainter& painter);
     void drawReplayOverlay(QPainter& painter);
 
     float    getTilePx() const;