Implement variance map shadows in interiors

[mchaptel]

Once the maps will have been figured out for exteriors, apply the same logic to the interior shadows.

[mchaptel]

VSM actually have a big issue linked to light bleeding.
A nice simple alternative is ESM, exponential Shadow Maps (as seen in https://developer.download.nvidia.com/presentations/2008/GDC/GDC08_SoftShadowMapping.pdf)

They only require a simple depth map but can still be filtered/blurred pre sampling. They are a nice improvement and have pretty good definition and stability.

changes here : mchaptel@25ee0a7

[mchaptel]

after some tests and implementing #87, turns out that using those algorithms doesn't produce as nice results and causes more problems. Fixing shadows inside required instead to properly account for lights attenuation on shadowed surfaces.