New params for Background Transparency

To fix issue: http://www.yafaray.org/node/468

New parameters added in the Render Tab to control the Background Transparency. These parameters have been applied to the Direct Lighting, Photon Mapping, SPPM and Path Tracing integrators. I did not change yet Bidir and Debug.

The new parameters are:

    Transp. Background: (true by default for backwards compatibility). When true, it renders the background as transparent. When false, it renders the background as opaque.
    Materials Transp. Refraction: (true by default for backwards compatibility). When true, the refractions of a transparent background are also transparent. When false, the refractions of a transparent background show the background as if it were opaque.