[Bf-codereview] Gamma Node for Cycles (issue 5488046)

[dfelinto at gmail.com]

Reviewers: bf-codereview_blender.org, brechtvl,

Description:
Gamma Node.

I tried to use powr instead of if (.. > 0) powf, but didn't work. No
harm I guess.
For after bcon5.

This node is not so useful for 'materials' but I believe it comes handy
for textures input. Although it's simple to do it with the math node.

Please review this at http://codereview.appspot.com/5488046/

Affected files:
   intern/cycles/app/cycles_xml.cpp
   intern/cycles/blender/blender_shader.cpp
   intern/cycles/kernel/CMakeLists.txt
   intern/cycles/kernel/kernel_types.h
   intern/cycles/kernel/osl/nodes/CMakeLists.txt
   intern/cycles/kernel/osl/nodes/node_gamma.osl
   intern/cycles/kernel/svm/svm.h
   intern/cycles/kernel/svm/svm_gamma.h
   intern/cycles/kernel/svm/svm_types.h
   intern/cycles/render/nodes.cpp
   intern/cycles/render/nodes.h
   source/blender/blenkernel/BKE_node.h
   source/blender/blenkernel/intern/node.c
   source/blender/makesdna/DNA_node_types.h
   source/blender/makesrna/intern/rna_nodetree.c
   source/blender/makesrna/intern/rna_nodetree_types.h
   source/blender/nodes/CMakeLists.txt
   source/blender/nodes/NOD_shader.h
   source/blender/nodes/shader/nodes/node_shader_gamma.c