Now that we use GS and textures array it's easier to add a cascade.
4 is still not optimal but I'd like to avoid cascade count inflation as
much as possible ; 4 x 1024x1024 has the bandwidth requirement of the
single 2048x2048 shadowmaps we previously had.
The 3 cascades are now rendered in a single pass, using 2D array texture and
a GS to do the dispatch ("layered rendering").
It's possible to use instancing instead but it requires the AMD_vertex_shader_layer
extension which is oddly part of opengl 4.2.
