Updated I420 video node to be consistent with RGB node implementation.

Don't keep the frame after it was uploaded to texture;
Upload frame data during rendering (in material shader updateState)
instead of updatePaintNode().

Change-Id: Ia6a4063bfa02d4d20f75735649fe4f777cf50494
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>
Reviewed-by: Jonas Rabbe <jonas.rabbe@nokia.com>

src/imports/multimedia/qsgvideonode_i420.h

@@ -60,13 +60,8 @@ public:
 private:
     void bindTexture(int id, int unit, int w, int h, const uchar *bits);
 
-    int m_width;
-    int m_height;
-    GLuint m_id[3];
-
     QVideoSurfaceFormat m_format;
     QSGVideoMaterial_YUV420 *m_material;
-    QVideoFrame m_frame;
 };
 
 class QSGVideoNodeFactory_I420 : public QSGVideoNodeFactory {