QVTKWidget vs QVTKWidget2 vs QVTKGraphicsItem.

For all of these it is impossible to draw Widget at the top of it using OSX (cocoa).

###QVTKWidget

Should Works on all plateforme, inherths from QWidget. QPixmap QPixmap::grabWidget() seemes to doesn't work with OSX if the widget is not visible.

###QVTKWidget2

QPixmap QPixmap::grabWidget() is not possible because it inherits from QGLWidget, you can use QImage QGLWidget::grabFrameBuffer() instead. This complicated the QImage medWorkspaceArea::grabScreenshot() method. ALso it doesn't work with OSX the resulting QImage is randomly filled.

###QVTKGraphicsItem

Allow to draw on the top of the view thanks to the QtGraphics framework. the widget returned by the view has to be a QGraphicsView. works well with some work (sync of the size of the item and the view, binds between Qt and VTK doubleClick event missing) under Linux and OSX, https://github.com/rdebroiz/medInria-public/tree/ContainerAndViewRefactoring-graphicsItem-windows but not with windows, where performances are not acceptable.

QPixmap QPixmap::grabWidget() is still not possible but we can use the QImage QGLFrmaeBufferObject::toImage() to get around (QVTKGraphicsItem, has a QGLFrmaeBufferObject as protected member).