Cycles Volume Render: work on nodes and closures.

* Henyey-Greenstein scattering closure implementation.
* Rename transparent to absorption node and isotropic to scatter node.
* Volume density is folded into the closure weights.
* OSL support for volume closures and nodes.
* This commit has no user visible changes, there is no volume render code yet.

This is work by "storm", Stuart Broadfoot, Thomas Dinges and myself.

source/blender/nodes/CMakeLists.txt

@@ -197,8 +197,8 @@ set(SRC
 	shader/nodes/node_shader_tex_sky.c
 	shader/nodes/node_shader_tex_voronoi.c
 	shader/nodes/node_shader_tex_wave.c
-	shader/nodes/node_shader_volume_isotropic.c
-	shader/nodes/node_shader_volume_transparent.c
+	shader/nodes/node_shader_volume_scatter.c
+	shader/nodes/node_shader_volume_absorption.c
 	shader/node_shader_tree.c
 	shader/node_shader_util.c