Cycles: Use curve approximation for blackbody instead of lookup table

Now we calculate color in range 800..12000 using an approximation a/x+bx+c for R and G and ((at + b)t + c)t + d) for B.
Max absolute error for RGB for non-lut function is less than 0.0001, which is enough to get the same 8 bit/channel color as for OSL with a noticeable performance difference.
However there is a slight visible difference between previous non-OSL implementation because of lookup table interpolation and offset-by-one mistake.
The previous implementation gave black color outside of soft range (t > 12000), now it gives the same color as for 12000.

Also blackbody node without input connected is being converted to value input at shader compile time.

Reviewers: dingto, sergey

Reviewed By: dingto

Subscribers: nutel, brecht, juicyfruit

Differential Revision: https://developer.blender.org/D1280

intern/cycles/render/svm.cpp

@@ -393,10 +393,6 @@ void SVMCompiler::generate_node(ShaderNode *node, set<ShaderNode*>& done)
 			current_shader->has_heterogeneous_volume = true;
 	}
 
-	/* detect if we have a blackbody converter, to prepare lookup table */
-	if(node->has_converter_blackbody())
-		current_shader->has_converter_blackbody = true;
-
 	if(node->has_object_dependency()) {
 		current_shader->has_object_dependency = true;
 	}
@@ -716,7 +712,6 @@ void SVMCompiler::compile(Shader *shader, vector<int4>& global_svm_nodes, int in
 	shader->has_surface_transparent = false;
 	shader->has_surface_bssrdf = false;
 	shader->has_bssrdf_bump = false;
-	shader->has_converter_blackbody = false;
 	shader->has_volume = false;
 	shader->has_displacement = false;
 	shader->has_heterogeneous_volume = false;