Inscribe StickWidget ellipse in square

Previously, it was not clear where the boundary of the StickWidget was when interacting outside of the circle. This aims to restore the gray square present in the Wx-era.

Source/Core/DolphinQt/TAS/StickWidget.cpp

@@ -47,6 +47,9 @@ void StickWidget::paintEvent(QPaintEvent* event)
 
   const int diameter = std::min(width(), height()) - PADDING * 2;
 
+  // inscribe the StickWidget inside a square
+  painter.fillRect(PADDING, PADDING, diameter, diameter, Qt::lightGray);
+
   painter.setBrush(Qt::white);
   painter.drawEllipse(PADDING, PADDING, diameter, diameter);