Implement PCSS (Percentage Closer Soft Shadows)

[mchaptel]

In order to have nice smooth shadows, we should look into ESM + PCSS shadows, to finally remove all remaining shadow map pixellation artefacts and diffuse shadows based on distance from occluder.

This seems like a nice reference

https://developer.download.nvidia.com/presentations/2008/GDC/GDC08_SoftShadowMapping.pdf