MythMainWindow: Only disable background autopainting for non-Qt painters

72976f3 disabled autopainting of the background with benefits
seen in most cases. On OSX with the Qt painter, however, the lack of
autopainting leads to some significant rendering 'dropouts' when
navigating between screens. As a compromise, retain autopainting for the
Qt painter.

mythtv/libs/libmythui/mythmainwindow.cpp

@@ -877,8 +877,6 @@ void MythMainWindow::Init(void)
 
     GetMythUI()->ThemeWidget(this);
     Show();
-    setAttribute(Qt::WA_NoSystemBackground);
-    setAutoFillBackground(false);
 
     if (!GetMythDB()->GetNumSetting("HideMouseCursor", 0))
         setMouseTracking(true); // Required for mouse cursor auto-hide
@@ -970,6 +968,12 @@ void MythMainWindow::Init(void)
         return;
     }
 
+    if (d->painter->GetName() != "Qt")
+    {
+        setAttribute(Qt::WA_NoSystemBackground);
+        setAutoFillBackground(false);
+    }
+
     d->paintwin->move(0, 0);
     d->paintwin->setFixedSize(size());
     d->paintwin->raise();