Cycles / Blackbody node:

* First (brute force) implementation for SVM. This works and delivers the same result as OSL, but it's slow. 
* Code inside svm_blackbody.h inspired by a patch by Philipp Oeser (#35698), thanks. 

Ideas:
* Use a lookup table to perform the calculations on render/ level.
* Implement it as a RNA property only, and do the calculation like Sun/Sky precompute.

intern/cycles/kernel/shaders/node_blackbody.osl

@@ -22,6 +22,10 @@ shader node_blackbody(
 	float Temperature = 1200.0,
 	output color Color = 0.0)
 {
-	Color = blackbody(Temperature);
+	color rgb = blackbody(Temperature);
+	
+	/* Scale by luminance */
+	float l = luminance(rgb);
+	Color = rgb /= l;
 }