Smoke (fire): Move spectrum code from C++ (intern/) to C code (BLI)

This change is for a few reasons:
- it works with color, and (therefore) will need to be color managed, at
some point. This will be much easier to do if the code is closer to the
actual color management code (in Blender's core, so to speak).
- it has nothing to do with the actual fire simulation, as it is just
used to create a lookup table
- it can be reused for other purposes (i.e. in Blender internal
renderer, if people are interrested in a blackbody node à la Cycles)
- cleanup: some functions (`contrain_rgb`, `xyz_to_rgb`) already exist
in BLI

Reviewers: brecht

Reviewed By: brecht

Subscribers: brecht

Differential Revision: https://developer.blender.org/D1719

source/blender/blenkernel/intern/smoke.c

@@ -128,7 +128,6 @@ void smoke_initBlenderRNA(struct FLUID_3D *UNUSED(fluid), float *UNUSED(alpha),
                           float *UNUSED(flame_vorticity), float *UNUSED(flame_ignition_temp), float *UNUSED(flame_max_temp)) {}
 struct DerivedMesh *smokeModifier_do(SmokeModifierData *UNUSED(smd), Scene *UNUSED(scene), Object *UNUSED(ob), DerivedMesh *UNUSED(dm), bool UNUSED(for_render)) { return NULL; }
 float smoke_get_velocity_at(struct Object *UNUSED(ob), float UNUSED(position[3]), float UNUSED(velocity[3])) { return 0.0f; }
-void flame_get_spectrum(unsigned char *UNUSED(spec), int UNUSED(width), float UNUSED(t1), float UNUSED(t2)) {}
 
 #endif /* WITH_SMOKE */