deprecate usage of QDesktopWidget for going fullscreen

Idea works as follows, while going fullscreen we compare the current window geometry with
available screens and ask for an intersection rectangle, we go fullscreen where most of
the window is located

GuessCurrentScreen could also potentially be used to see which screen
the window is on for dynamic DPI handling

src/yuzu/main.cpp

@@ -52,7 +52,6 @@ static FileSys::VirtualFile VfsDirectoryCreateFileWrapper(const FileSys::Virtual
 #define QT_NO_OPENGL
 #include <QClipboard>
 #include <QDesktopServices>
-#include <QDesktopWidget>
 #include <QFile>
 #include <QFileDialog>
 #include <QInputDialog>
@@ -60,6 +59,7 @@ static FileSys::VirtualFile VfsDirectoryCreateFileWrapper(const FileSys::Virtual
 #include <QProgressBar>
 #include <QProgressDialog>
 #include <QPushButton>
+#include <QScreen>
 #include <QShortcut>
 #include <QStatusBar>
 #include <QString>
@@ -1002,7 +1002,7 @@ void GMainWindow::InitializeHotkeys() {
 
 void GMainWindow::SetDefaultUIGeometry() {
     // geometry: 53% of the window contents are in the upper screen half, 47% in the lower half
-    const QRect screenRect = QApplication::desktop()->screenGeometry(this);
+    const QRect screenRect = QGuiApplication::primaryScreen()->geometry();
 
     const int w = screenRect.width() * 2 / 3;
     const int h = screenRect.height() * 2 / 3;
@@ -2581,6 +2581,18 @@ void GMainWindow::ToggleFullscreen() {
     }
 }
 
+// We're going to return the screen that the given window has the most pixels on
+static QScreen* GuessCurrentScreen(QWidget* window) {
+    const QList<QScreen*> screens = QGuiApplication::screens();
+    return *std::max_element(
+        screens.cbegin(), screens.cend(), [window](const QScreen* left, const QScreen* right) {
+            const QSize left_size = left->geometry().intersected(window->geometry()).size();
+            const QSize right_size = right->geometry().intersected(window->geometry()).size();
+            return (left_size.height() * left_size.width()) <
+                   (right_size.height() * right_size.width());
+        });
+}
+
 void GMainWindow::ShowFullscreen() {
     const auto show_fullscreen = [](QWidget* window) {
         if (Settings::values.fullscreen_mode.GetValue() == Settings::FullscreenMode::Exclusive) {
@@ -2589,7 +2601,7 @@ void GMainWindow::ShowFullscreen() {
         }
         window->hide();
         window->setWindowFlags(window->windowFlags() | Qt::FramelessWindowHint);
-        const auto screen_geometry = QApplication::desktop()->screenGeometry(window);
+        const auto screen_geometry = GuessCurrentScreen(window)->geometry();
         window->setGeometry(screen_geometry.x(), screen_geometry.y(), screen_geometry.width(),
                             screen_geometry.height() + 1);
         window->raise();