light passes: barebones, but not working

see description of the problem in
kernel_displace.h :: compute_light_pass()
initialize RNG and other small changes as per Brecht suggestion
force use_light_pass to be on when baking light passes
make sure film use_light_visibility is on since blender_session (just to be safe)
Passing samples, and hack to enhance bake result

To see the result pick a scene render sample odd (e.g., 27, 39, ...).
Only working-ish for diffuse only scene (no emmit and no lights).
Cycles Bake fixes:

* Add code for summing multiple AA samples in PathRadiance, see path_radiance_accum_sample.
* Move #includes out of namespace to avoid having to use ::ccl::ccl
* Fix wrong initialization of RNG with hash,
* Fix light computation loop.
* Initialize incoming vector with Ng instead of zero, better not have degenerate value here.
* Use shader_emissive_eval to simplify emission pass code.

* Write files in PNG instead of JPEG format.
* Also enable linear => sRGB conversion. It should actually check if it's a
  non-color data pass and avoid conversion then, but this is needed for inspecting
  light passes.
always write alpha 1.0 for now
remove old code for passes + style touch ups
save as linear if is a data pass type
AO pass
Merge branch 'bake-cycles-dev' into bake-cycles

* Light Passes Baking
* Saving as PNG
* Handling color-space based on passtype