* Volume Rendering: Voxel data

This commit introduces a new texture ('Voxel Data'), used to load up saved voxel 
data sets for rendering, contributed by Raúl 'farsthary' Fernández Hernández 
with some additional tweaks. Thanks, Raúl!

The texture works similar to the existing point density texture, currently it 
only provides intensity information, which can then be mapped (for example) to 
density in a volume material. This is an early version, intended to read the 
voxel format saved by Raúl's command line simulators, in future revisions 
there's potential for making a more full-featured 'Blender voxel file format', 
and also for supporting other formats too.

Note: Due to some subtleties in Raúl's existing released simulators, in  order 
to load them correctly the voxel data texture, you'll need to raise the 
'resolution' value by 2. So if you baked out the simulation at resolution 50, 
enter 52 for the resolution in the texture panel. This can possibly be fixed in 
the simulator later on.

Right now, the way the texture is mapped is just in the space 0,0,0 <-> 1,1,1 
and it can appear rotated 90 degrees incorrectly. This will be tackled, for now, 
probably the easiest way to map it is with and empty, using Map Input -> Object.

Smoke test: http://www.vimeo.com/2449270

One more note, trilinear interpolation seems a bit slow at the moment, we'll 
look into this.

For curiosity, while testing/debugging this, I made a script that exports a mesh 
to voxel data. Here's a test of grogan (www.kajimba.com) converted to voxels, 
rendered as a volume: http://www.vimeo.com/2512028

The script is available here: http://mke3.net/projects/bpython/export_object_voxeldata.py

* Another smaller thing, brought back early ray termination (was disabled 
previously for debugging) and made it user configurable. It now appears as a new 
value in the volume material: 'Depth Cutoff'. For some background info on what 
this does, check:
http://farsthary.wordpress.com/2008/12/11/cutting-down-render-times/

* Also some disabled work-in-progess code for light cache

source/blender/blenloader/intern/writefile.c

@@ -1339,6 +1339,7 @@ static void write_textures(WriteData *wd, ListBase *idbase)
 				writestruct(wd, DATA, "PointDensity", 1, tex->pd);
 				if(tex->pd->coba) writestruct(wd, DATA, "ColorBand", 1, tex->pd->coba);
 			}
+			if(tex->vd) writestruct(wd, DATA, "VoxelData", 1, tex->vd);
 			
 			/* nodetree is integral part of texture, no libdata */
 			if(tex->nodetree) {