Add video filtering support to VideoOutput

Add the QAbstractVideoFilter base class and integrate it with VideoOutput.

This can be used to perform arbitrary filtering or image processing
on the frames of a video stream of a VideoOutput element right before
the OpenGL texture is provided to the scenegraph by the video node.

This opens up the possibility to integrate computer vision
frameworks or accelerated image processing with Qt Quick applications
that display video streams using Qt Multimedia.

Conceptually it is somewhat similar to QVideoProbe, this
approach however allows modifying the frame, in real time
with tight integration to the scenegraph node, and targets
Qt Quick meaning setting up the filter and processing the results
of the computations happen completely in QML.

[ChangeLog] Added QAbstractVideoFilter that serves as a base class for QML
video filtering elements that integrate compute, vision, and image processing
frameworks with VideoOutput.

Change-Id: Ice1483f8c2daec5a43536978627a7bbb64549480
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>

src/qtmultimediaquicktools/qdeclarativevideooutput_render_p.h

@@ -48,6 +48,8 @@ QT_BEGIN_NAMESPACE
 class QSGVideoItemSurface;
 class QVideoRendererControl;
 class QOpenGLContext;
+class QAbstractVideoFilter;
+class QVideoFilterRunnable;
 
 class QDeclarativeVideoRendererBackend : public QDeclarativeVideoBackend
 {
@@ -70,7 +72,14 @@ public:
     void present(const QVideoFrame &frame);
     void stop();
 
+    void appendFilter(QAbstractVideoFilter *filter) Q_DECL_OVERRIDE;
+    void clearFilters() Q_DECL_OVERRIDE;
+    void releaseResources() Q_DECL_OVERRIDE;
+    void invalidateSceneGraph() Q_DECL_OVERRIDE;
+
 private:
+    void scheduleDeleteFilterResources();
+
     QPointer<QVideoRendererControl> m_rendererControl;
     QList<QSGVideoNodeFactoryInterface*> m_videoNodeFactories;
     QSGVideoItemSurface *m_surface;
@@ -83,6 +92,14 @@ private:
     QMutex m_frameMutex;
     QRectF m_renderedRect;         // Destination pixel coordinates, clipped
     QRectF m_sourceTextureRect;    // Source texture coordinates
+
+    struct Filter {
+        Filter() : filter(0), runnable(0) { }
+        Filter(QAbstractVideoFilter *filter) : filter(filter), runnable(0) { }
+        QAbstractVideoFilter *filter;
+        QVideoFilterRunnable *runnable;
+    };
+    QList<Filter> m_filters;
 };
 
 class QSGVideoItemSurface : public QAbstractVideoSurface