Cycles: Point density texture support

This commit implements point density texture for Cycles shading nodes.

It's done via creating voxel texture at shader compilation time, Not
totally memory efficient, but avoids adding sampling code to kernel
(which keeps render time as low as possible), In the future this will
be compensated by using OpenVDB for more efficient storage of sparse
volume data.

Sampling of the voxel texture is happening at blender side and the
same code is used as for Blender Internal's renderer.

This texture is controlled by only object, particle system and radius.
Linear falloff is used and there's no turbulence. This is because
falloff is expected to happen using Curve Mapping node. Turbulence
will be done as a distortion on the input coordinate. It's already
possible to fake it using nose textures and in the future we can add
more proper turbulence distortion node, which then could also be used
for 2D texture mapping.

Particle color support is done by Lukas, thanks!

release/scripts/startup/nodeitems_builtins.py

@@ -237,6 +237,7 @@ shader_node_categories = [
         NodeItem("ShaderNodeTexMagic"),
         NodeItem("ShaderNodeTexChecker"),
         NodeItem("ShaderNodeTexBrick"),
+        NodeItem("ShaderNodeTexPointDensity"),
         ]),
     ShaderNewNodeCategory("SH_NEW_OP_COLOR", "Color", items=[
         NodeItem("ShaderNodeMixRGB"),