Manually center the window after setting its size on macOS

Qt's default window positioning is somewhat unreliable, and since we
don't currently remember the window position, let's at least make sure
that it's properly centered on open.

src/frontend/qt_sdl/main.cpp

@@ -1566,6 +1566,13 @@ MainWindow::MainWindow(QWidget* parent) : QMainWindow(parent)
 
     resize(Config::WindowWidth, Config::WindowHeight);
 
+#ifdef Q_OS_MAC
+    QPoint screenCenter = screen()->availableGeometry().center();
+    QRect frameGeo = frameGeometry();
+    frameGeo.moveCenter(screenCenter);
+    move(frameGeo.topLeft());
+#endif
+
     if (oldMax)
         showMaximized();
     else