[wayland] Create QPainterBackend through the AbstractBackend

SceneQPainter delegates creation to the AbstractBackend removing the
casting logic.

abstract_backend.cpp

@@ -54,4 +54,9 @@ OpenGLBackend *AbstractBackend::createOpenGLBackend()
     return nullptr;
 }
 
+QPainterBackend *AbstractBackend::createQPainterBackend()
+{
+    return nullptr;
+}
+
 }

abstract_backend.h

@@ -26,6 +26,7 @@ namespace KWin
 {
 
 class OpenGLBackend;
+class QPainterBackend;
 class Screens;
 
 class KWIN_EXPORT AbstractBackend : public QObject
@@ -38,6 +39,7 @@ public:
     virtual void installCursorImage(Qt::CursorShape shape);
     virtual Screens *createScreens(QObject *parent = nullptr);
     virtual OpenGLBackend *createOpenGLBackend();
+    virtual QPainterBackend *createQPainterBackend();
 
 protected:
     explicit AbstractBackend(QObject *parent = nullptr);

scene_qpainter.cpp

@@ -256,10 +256,9 @@ SceneQPainter *SceneQPainter::createScene(QObject *parent)
     QScopedPointer<QPainterBackend> backend;
 #if HAVE_WAYLAND
     if (kwinApp()->shouldUseWaylandForCompositing()) {
-        if (X11WindowedBackend *b = dynamic_cast<X11WindowedBackend*>(waylandServer()->backend())) {
-            backend.reset(new X11WindowedQPainterBackend(b));
-        } else if (Wayland::WaylandBackend *b = dynamic_cast<Wayland::WaylandBackend*>(waylandServer()->backend())) {
-            backend.reset(new WaylandQPainterBackend(b));
+        backend.reset(waylandServer()->backend()->createQPainterBackend());
+        if (backend.isNull()) {
+            return nullptr;
         }
         if (backend->isFailed()) {
             return NULL;

wayland_backend.cpp

@@ -23,6 +23,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include "cursor.h"
 #include "input.h"
 #include "main.h"
+#include "scene_qpainter.h"
 #include "screens_wayland.h"
 #include "utils.h"
 #include "wayland_server.h"
@@ -657,6 +658,11 @@ OpenGLBackend *WaylandBackend::createOpenGLBackend()
 #endif
 }
 
+QPainterBackend *WaylandBackend::createQPainterBackend()
+{
+    return new WaylandQPainterBackend(this);
+}
+
 }
 
 } // KWin

wayland_backend.h

@@ -171,6 +171,7 @@ public:
 
     Screens *createScreens(QObject *parent = nullptr) override;
     OpenGLBackend *createOpenGLBackend() override;
+    QPainterBackend *createQPainterBackend() override;
 
 protected:
     void connectNotify(const QMetaMethod &signal) override;

x11windowed_backend.cpp

@@ -20,6 +20,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include "x11windowed_backend.h"
 #include "composite.h"
 #include "input.h"
+#include "scene_qpainter.h"
 #include "screens_x11windowed.h"
 #include "utils.h"
 #include "wayland_server.h"
@@ -314,4 +315,9 @@ OpenGLBackend *X11WindowedBackend::createOpenGLBackend()
     return nullptr;
 }
 
+QPainterBackend *X11WindowedBackend::createQPainterBackend()
+{
+    return new X11WindowedQPainterBackend(this);
+}
+
 }

x11windowed_backend.h

@@ -68,6 +68,7 @@ public:
 
     Screens *createScreens(QObject *parent = nullptr) override;
     OpenGLBackend *createOpenGLBackend() override;
+    QPainterBackend* createQPainterBackend() override;
 
 Q_SIGNALS:
     void sizeChanged();