WorkAreaTarget_Cairo_Tile: when the tile is added destroy the Cairo context
When resize the work area clear the cairo book too.
Initialize refreshes to zero at creation
When calculating the next unrendered tile consider if uses Cairo render
add_tile now receives a reference to a cairo_surface_t pointer
Gamma filter the surface tile before store it at cairo_book
New class AsyncTarget_Cairo_Tile to render the tiles asynchronously .