Brightness/Contrast Node for Cycles

Contrast helps to adjust IBL (HDR images used for background lighting).
Note: In the UI we are caling it Bright instead of Brightness. This copy what Blender composite is doing.
Note2: the algorithm we are using produces pure black when contrast is 100. I'm not a fan of that, but it's a division by zero. I would like to look at other algorithms (what gimp does for example). But that would be only after 2.62.

intern/cycles/kernel/svm/svm.h

@@ -131,6 +131,7 @@ CCL_NAMESPACE_END
 #include "svm_hsv.h"
 #include "svm_image.h"
 #include "svm_gamma.h"
+#include "svm_brightness.h"
 #include "svm_invert.h"
 #include "svm_light_path.h"
 #include "svm_magic.h"
@@ -270,6 +271,9 @@ __device_noinline void svm_eval_nodes(KernelGlobals *kg, ShaderData *sd, ShaderT
 			case NODE_GAMMA:
 				svm_node_gamma(sd, stack, node.y, node.z, node.w);
 				break;
+			case NODE_BRIGHTCONTRAST:
+				svm_node_brightness(sd, stack, node.y, node.z, node.w);
+				break;
 			case NODE_MIX:
 				svm_node_mix(kg, sd, stack, node.y, node.z, node.w, &offset);
 				break;