Fix for bug #1791

- onlyshadow material defaulted to black when no shadow calc was used, is
  now 100% transparent
- AO 'shadows' were not included in onlyshadow material
- alpha render appeared to be wrong since 2.32... it was gamma corrected
  giving difference in OSA render with 'Gamma' on

That alpha issue i am going to tackle once, it is not well functioning, and
might be combined with new 'transmission' colors idea