Cascaded shadow map.

This shadow mapping technique is used for directional lights.
It generates a given number of shadow maps for ranges of distance.

Resources :
https://msdn.microsoft.com/en-us/library/windows/desktop/ee416307(v=vs.85).aspx
http://developer.download.nvidia.com/SDK/10.5/opengl/src/cascaded_shadow_maps/doc/cascaded_shadow_maps.pdf
http://www.asawicki.info/news_1283_cascaded_shadow_mapping.html
~ http://http.developer.nvidia.com/GPUGems3/gpugems3_ch10.html

@author Johann Sorel

param  camera scene camera
param  light directional light generating the shadows
param  nbShadows number of shadow maps to generate
param  maxDistance max shadow distance, no shadow map will be generated further