Modes API for QPlatformScreen

Add an API for modes (that is screen size and refresh rate).

This will allow platform plugins to list modes available
for a screen.

[ChangeLog][QtGui][QPA] Add an API for modes.

Change-Id: I91851c51cc60a1544465dfa3b4d96cc667237a0a
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/gui/kernel/qplatformscreen.cpp

@@ -533,4 +533,53 @@ void QPlatformScreen::setPowerState(PowerState state)
     Q_UNUSED(state);
 }
 
+/*!
+    Reimplement this function in subclass to return the list
+    of modes for this screen.
+
+    The default implementation returns a list with
+    only one mode from the current screen size and refresh rate.
+
+    \sa QPlatformScreen::geometry
+    \sa QPlatformScreen::refreshRate
+
+    \since 5.9
+*/
+QVector<QPlatformScreen::Mode> QPlatformScreen::modes() const
+{
+    QVector<QPlatformScreen::Mode> list;
+    list.append({geometry().size(), refreshRate()});
+    return list;
+}
+
+/*!
+    Reimplement this function in subclass to return the
+    index of the current mode from the modes list.
+
+    The default implementation returns 0.
+
+    \sa QPlatformScreen::modes
+
+    \since 5.9
+*/
+int QPlatformScreen::currentMode() const
+{
+    return 0;
+}
+
+/*!
+    Reimplement this function in subclass to return the preferred
+    mode index from the modes list.
+
+    The default implementation returns 0.
+
+    \sa QPlatformScreen::modes
+
+    \since 5.9
+*/
+int QPlatformScreen::preferredMode() const
+{
+    return 0;
+}
+
 QT_END_NAMESPACE

src/gui/kernel/qplatformscreen.h

@@ -95,6 +95,11 @@ public:
         PowerStateOff
     };
 
+    struct Mode {
+        QSize size;
+        qreal refreshRate;
+    };
+
     QPlatformScreen();
     virtual ~QPlatformScreen();
 
@@ -139,6 +144,11 @@ public:
     virtual PowerState powerState() const;
     virtual void setPowerState(PowerState state);
 
+    virtual QVector<Mode> modes() const;
+
+    virtual int currentMode() const;
+    virtual int preferredMode() const;
+
     static int angleBetween(Qt::ScreenOrientation a, Qt::ScreenOrientation b);
     static QTransform transformBetween(Qt::ScreenOrientation a, Qt::ScreenOrientation b, const QRect &target);
     static QRect mapBetween(Qt::ScreenOrientation a, Qt::ScreenOrientation b, const QRect &rect);