[aurorae] Hack to get Aurorae work with compositing

This is a temporary solution! A proper solution needs changes in
libkdecoration and paint redirector.

The hack redirects the rendering into an FBO. After each rendering we
get a QImage from the FBO and store that in a buffer. As we unfortunately
do not know what changed, we schedule a complete update on the deco's
widget. Once we get the paint event we just render the buffer on the
widget. And thus we have copied the content to a place where it
integrates with the paint redirector.

As already written, this is a horrible hack and I'm not proud about it.
There are just too many copies involved.

So how to improve?
* deco should be able to just provide a QImage to the PaintRedirector
  without the paint to the QWidget.
* only do the FBO -> QImage step if it is needed by the compositor, that
  is compress the events
* for OpenGL compositing it would be totally awesome if we could just
  make the contexts sharing so that we can just reuse the texture from
  the FBO directly.

clients/aurorae/src/aurorae.cpp

@@ -21,6 +21,8 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 #include <QApplication>
 #include <QDebug>
+#include <QOpenGLFramebufferObject>
+#include <QPainter>
 #include <QQmlComponent>
 #include <QQmlContext>
 #include <QQmlEngine>
@@ -259,6 +261,24 @@ void AuroraeClient::init()
     widget()->setAttribute(Qt::WA_NoSystemBackground);
     widget()->installEventFilter(this);
     m_view = new QQuickWindow();
+    if (compositingActive()) {
+        connect(m_view, &QQuickWindow::beforeRendering, [this]() {
+            if (m_fbo.isNull() || m_fbo->size() != QSize(width(), height())) {
+                m_fbo.reset(new QOpenGLFramebufferObject(QSize(width(), height()), QOpenGLFramebufferObject::CombinedDepthStencil));
+                if (!m_fbo->isValid()) {
+                    qWarning() << "Creating FBO as render target failed";
+                    m_fbo.reset();
+                    return;
+                }
+            }
+            m_view->setRenderTarget(m_fbo.data());
+        });
+        connect(m_view, &QQuickWindow::afterRendering, [this]{
+            m_buffer = m_fbo->toImage();
+        });
+        connect(m_view, &QQuickWindow::afterRendering, widget(),
+                static_cast<void (QWidget::*)(void)>(&QWidget::update), Qt::QueuedConnection);
+    }
     m_view->setColor(Qt::transparent);
     m_container = QWidget::createWindowContainer(m_view, widget(), Qt::X11BypassWindowManagerHint);
     m_container->setAttribute(Qt::WA_TranslucentBackground);
@@ -275,6 +295,11 @@ void AuroraeClient::init()
 
 bool AuroraeClient::eventFilter(QObject *object, QEvent *event)
 {
+    if (compositingActive() && !m_fbo.isNull() && object == widget() && QEvent::Paint) {
+        QPainter painter(widget());
+        painter.drawImage(QPoint(0, 0), m_buffer);
+        return false;
+    }
     // we need to filter the wheel events on the decoration
     // QML does not yet provide a way to accept wheel events, this will change with Qt 5
     // TODO: remove in KDE5

clients/aurorae/src/aurorae.h

@@ -23,6 +23,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include <kdecoration.h>
 #include <kdecorationfactory.h>
 
+class QOpenGLFramebufferObject;
 class QQmlComponent;
 class QQmlEngine;
 class QQuickItem;
@@ -160,6 +161,8 @@ private:
     QQuickWindow *m_view;
     QQuickItem *m_item;
     QWidget *m_container;
+    QScopedPointer<QOpenGLFramebufferObject> m_fbo;
+    QImage m_buffer;
 };
 
 }