Fixes that billboards were falsely treated as transparent objects - which they are not (they us alpha reject) and adds refractive water effect for default water blocks - aside various other water related tweaks. Most likely fixes #537.

* Water blocks are now translucent again using a slightly refractive effect (using the default texture for perturbation)
* Fixes that billboards were treated as non-opaque (so they were not rendered to the opaque scene rtt (and were thus not visible while looking through blend materials - like water)
* Removes that water became opaque while swimming - this is no longer needed
* Fixes that the fresnel term used for blending between the reflection and refraction target could become invalid
* Changes how swimming and water rendering is treated