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/CMakeLists.txt

@@ -18,7 +18,6 @@ set(SRC
 	attribute.cpp
 	background.cpp
 	bake.cpp
-	blackbody.cpp
 	buffers.cpp
 	camera.cpp
 	film.cpp
@@ -47,7 +46,6 @@ set(SRC_HEADERS
 	attribute.h
 	bake.h
 	background.h
-	blackbody.h
 	buffers.h
 	camera.h
 	film.h