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.