Add a checkbox to use Cairo render a the Preview dialog.
New class Preview Target Cairo
Redefine clear() member to take care on destroy the references to the cairo surfaces. Don't directly call the resize member without have cleared it before
Notify subscribers that the frame has been finished and placed on the FlipBook
Dialog Preview can pass down the use_cairo option
When destroying Preview, remove the references of the surfaces first.
When removing the surface references check if the pointer has been initialized to NULL first.
Initialize the cairo surface pointer to NULL when working with normal render.
Turn FlipBookElem into a class to allow control on constructor and destructor
Effectively use the Cairo render on the Preview when the Use Cairo render option is checked. It doesn't scale up or centers the rendered image yet.