Generate a pvs for refraction renders just like for reflection renders, this fixes...

Generate a pvs for refraction renders just like for reflection renders, this fixes a water rendering issue in the Doombringer map duel2.bsp where certain leafs did not include the necessary pvs data (or something was wrong with how the renderer used the existing vis data).

In light of the above, allow r_wateralpha when r_water is used, even if the bsp is not watervised - this works for all non-recursive cases (so the elder hall water pool in start.bsp is still broken).

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12411 d7cf8633-e32d-0410-b094-e92efae38249