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/render/nodes.h

@@ -320,6 +320,11 @@ public:
 	SHADER_NODE_CLASS(GammaNode)
 };
 
+class BrightContrastNode : public ShaderNode {
+public:
+	SHADER_NODE_CLASS(BrightContrastNode)
+};
+
 class SeparateRGBNode : public ShaderNode {
 public:
 	SHADER_NODE_CLASS(SeparateRGBNode)