Add classes PCLContextItem and PCLContextImageItem

PCLContextItem inherits from vtkImageContext and is the basic class of 2D shapes
PCLContextImageItem inherits from vtkImageContext and hold a vtkImageData*
member
Fix ImageViewer overlay issue

vtkImageCanvasSource2D is not suited for overlays since the opacity settings
affect the whole image so it would be better for painting. vtkContextItem
combined with vtkContext2D is better since it allows more flexibility and
overlays are correctly displayed without affecting the background cloud image.