yuzu/bootmanager: Remove pointer downcast in GRenderWindow's constructor

We can just pass a pointer to GMainWindow directly and make it a
requirement of the interface. This makes the interface a little safer,
since this would technically otherwise allow any random QWidget to be
the parent of a render window, downcasting it to GMainWindow (which is
undefined behavior).

src/yuzu/bootmanager.cpp

@@ -185,7 +185,7 @@ private:
     bool do_painting;
 };
 
-GRenderWindow::GRenderWindow(QWidget* parent, EmuThread* emu_thread)
+GRenderWindow::GRenderWindow(GMainWindow* parent, EmuThread* emu_thread)
     : QWidget(parent), emu_thread(emu_thread) {
     setWindowTitle(QStringLiteral("yuzu %1 | %2-%3")
                        .arg(QString::fromUtf8(Common::g_build_name),
@@ -194,8 +194,7 @@ GRenderWindow::GRenderWindow(QWidget* parent, EmuThread* emu_thread)
     setAttribute(Qt::WA_AcceptTouchEvents);
 
     InputCommon::Init();
-    connect(this, &GRenderWindow::FirstFrameDisplayed, static_cast<GMainWindow*>(parent),
-            &GMainWindow::OnLoadComplete);
+    connect(this, &GRenderWindow::FirstFrameDisplayed, parent, &GMainWindow::OnLoadComplete);
 }
 
 GRenderWindow::~GRenderWindow() {

src/yuzu/bootmanager.h

@@ -114,7 +114,7 @@ class GRenderWindow : public QWidget, public Core::Frontend::EmuWindow {
     Q_OBJECT
 
 public:
-    GRenderWindow(QWidget* parent, EmuThread* emu_thread);
+    GRenderWindow(GMainWindow* parent, EmuThread* emu_thread);
     ~GRenderWindow() override;
 
     // EmuWindow implementation