Add Raytracer fluid detection

[Pecant-Pie]

As far as I can tell there is no way to detect fluids (at least not water) with the raytracer.
I think this is a pretty important feature for moving vehicles, and also would just be nice to take better landscape pictures.

Having it as an option (similar to check_for_blocks_in_world) to the raytrace function would be very helpful.

[SuperSpaceEye]

yea, i will eventually add a detection for fluid