Fix the screen going black on pause after changing windows

More info: https://zeduckmaster.frama.io/2016/how-to-create-a-custom-rendering-in-a-qt5-widget/

Source/Core/DolphinQt/RenderWidget.cpp

@@ -52,6 +52,7 @@ RenderWidget::RenderWidget(QWidget* parent) : QWidget(parent)
   });
 
   connect(&Settings::Instance(), &Settings::EmulationStateChanged, this, [this](Core::State state) {
+    // Stop filling the background once emulation starts, but fill it until then (Bug 10958)
     SetFillBackground(Config::Get(Config::MAIN_RENDER_TO_MAIN) &&
                       state == Core::State::Uninitialized);
     if (state == Core::State::Running)
@@ -90,9 +91,15 @@ RenderWidget::RenderWidget(QWidget* parent) : QWidget(parent)
 
 void RenderWidget::SetFillBackground(bool fill)
 {
+  setAutoFillBackground(fill);
   setAttribute(Qt::WA_OpaquePaintEvent, !fill);
   setAttribute(Qt::WA_NoSystemBackground, !fill);
-  setAutoFillBackground(fill);
+  setAttribute(Qt::WA_PaintOnScreen, !fill);
+}
+
+QPaintEngine* RenderWidget::paintEngine() const
+{
+  return autoFillBackground() ? QWidget::paintEngine() : nullptr;
 }
 
 void RenderWidget::dragEnterEvent(QDragEnterEvent* event)

Source/Core/DolphinQt/RenderWidget.h

@@ -19,6 +19,7 @@ public:
 
   bool event(QEvent* event) override;
   void showFullScreen();
+  QPaintEngine* paintEngine() const override;
 
 signals:
   void EscapePressed();