[wayland] BasicScreens can serve multiple screens

For this AbstractBackend has a new virtual screenGeometries() method
which returns a QVector<QRect>. By default it's just one QRect at 0/0
with the size of the one screenSize().

abstract_backend.cpp

@@ -335,4 +335,9 @@ QSize AbstractBackend::screenSize() const
     return QSize();
 }
 
+QVector<QRect> AbstractBackend::screenGeometries() const
+{
+    return QVector<QRect>({QRect(QPoint(0, 0), screenSize())});
+}
+
 }

abstract_backend.h

@@ -67,6 +67,13 @@ public:
      * Base implementation returns an invalid size.
      **/
     virtual QSize screenSize() const;
+    /**
+     * Implementing subclasses should provide all geometries in case the backend represents
+     * a basic screen and uses the BasicScreens.
+     *
+     * Base implementation returns one QRect positioned at 0/0 with screenSize() as size.
+     **/
+    virtual QVector<QRect> screenGeometries() const;
 
     bool usesSoftwareCursor() const {
         return m_softWareCursor;

screens.cpp

@@ -205,29 +205,45 @@ void BasicScreens::init()
 
 QRect BasicScreens::geometry(int screen) const
 {
-    if (screen == 0) {
-        return QRect(QPoint(0, 0), size(screen));
+    if (screen < m_geometries.count()) {
+        return m_geometries.at(screen);
     }
     return QRect();
 }
 
 QSize BasicScreens::size(int screen) const
 {
-    if (screen == 0) {
-        return m_backend->screenSize();
+    if (screen < m_geometries.count()) {
+        return m_geometries.at(screen).size();
     }
     return QSize();
 }
 
 void BasicScreens::updateCount()
 {
-    setCount(1);
+    m_geometries = m_backend->screenGeometries();
+    setCount(m_geometries.count());
 }
 
 int BasicScreens::number(const QPoint &pos) const
 {
-    Q_UNUSED(pos)
-    return 0;
+    int bestScreen = 0;
+    int minDistance = INT_MAX;
+    for (int i = 0; i < m_geometries.count(); ++i) {
+        const QRect &geo = m_geometries.at(i);
+        if (geo.contains(pos)) {
+            return i;
+        }
+        int distance = QPoint(geo.topLeft() - pos).manhattanLength();
+        distance = qMin(distance, QPoint(geo.topRight() - pos).manhattanLength());
+        distance = qMin(distance, QPoint(geo.bottomRight() - pos).manhattanLength());
+        distance = qMin(distance, QPoint(geo.bottomLeft() - pos).manhattanLength());
+        if (distance < minDistance) {
+            minDistance = distance;
+            bestScreen = i;
+        }
+    }
+    return bestScreen;
 }
 
 } // namespace

screens.h

@@ -29,6 +29,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include <QObject>
 #include <QRect>
 #include <QTimer>
+#include <QVector>
 
 namespace KWin
 {
@@ -167,6 +168,7 @@ public:
 
 private:
     AbstractBackend *m_backend;
+    QVector<QRect> m_geometries;
 };
 
 inline