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.

intern/cycles/render/nodes.h

@@ -316,14 +316,14 @@ public:
 	ClosureType closure;
 };
 
-class TransparentVolumeNode : public VolumeNode {
+class AbsorptionVolumeNode : public VolumeNode {
 public:
-	SHADER_NODE_CLASS(TransparentVolumeNode)
+	SHADER_NODE_CLASS(AbsorptionVolumeNode)
 };
 
-class IsotropicVolumeNode : public VolumeNode {
+class ScatterVolumeNode : public VolumeNode {
 public:
-	SHADER_NODE_CLASS(IsotropicVolumeNode)
+	SHADER_NODE_CLASS(ScatterVolumeNode)
 };
 
 class HairBsdfNode : public BsdfNode {