Qt: Fix Wayland support

Qt will only create a Wayland surface when show() is called on the main
window and on the ScreenWidget. Thus, call the function before creating
the GL context.

Doesn't cause regressions on XWayland, untested in other platforms.

Fixes #586

src/panda_qt/main_window.cpp

@@ -22,6 +22,7 @@ MainWindow::MainWindow(QApplication* app, QWidget* parent) : QMainWindow(parent)
 	// Enable drop events for loading ROMs
 	setAcceptDrops(true);
 	resize(800, 240 * 4);
+	show();
 
 	// We pass a callback to the screen widget that will be triggered every time we resize the screen
 	screen = new ScreenWidget([this](u32 width, u32 height) { handleScreenResize(width, height); }, this);
@@ -699,4 +700,4 @@ void MainWindow::setupControllerSensors(SDL_GameController* controller) {
 	if (haveAccelerometer) {
 		SDL_GameControllerSetSensorEnabled(controller, SDL_SENSOR_ACCEL, SDL_TRUE);
 	}
-}
+}