Cycles-Bake: placeholder for Sub-Surface-Scattering

Talking with Thomas Ding he said that "CLOSURE_IS_BSDF_BSSRDF" is what we use for Render
Passes accumulation. Which means we may need to run different functions
for SSS. For now leaving it as it is, in wait of more capable hands.

kernel_subsurface.h ~152
 /* replace CLOSURE_BSDF_DIFFUSE_ID with this special ID so render passes
  * can recognize it as not being a regular diffuse closure */

  sc->type = CLOSURE_BSDF_BSSRDF_ID;
Cycles-Bake: SSS attempt

Im getting something, but it seems a bit different from the results from
Render Layers. I did something similar with what we did for AO