Blender Git Loki

Cycles: Experiment with native cubemap support in Cycles

This commit implements a native cubemap rendering in Cycles,
which is currently a research project with the following goals:

- Make it easier to render cubemaps for engines which supports
or expects them to be used.

- Intermediate projection to optimize rendering of equirectangular
camera.

Cube map layout is not currently standart but it's most efficient
from the resolution and storage point of views and it's currently:

+-------+-------+--------+
| Back | Right | Bottom |
+-------+-------+--------+
| Front | Left | Top |
+-------+-------+--------+

It's easy to re-map views in the future.

There are still issues to be solved, but it'll be nice to get
VR and panorama artists involved into feedback already. So the
limitations are:

- There's no auto-remapping to equirectangular map happening yet,
so the only way to see result is to put result to an environment
map with cubemap projection and either re-render or investigate
it in viewport.

- Projection formulas are not optimized by any mean. This is to
keep them totally obvious to make it easier to make further
tweaks.