improve shadow quality with sparse vsm

[octanejohn]

this idea should be for near field
https://ktstephano.github.io/rendering/stratusgfx/svsm

for far field sdf shadows could be used but are too limiting

another idea i am trying to see if it works with far shadows is this(maybe works with other ideas too? gi, ao, far reflection, gpu occlution culling?)
https://zheng95z.github.io/publications/roma23

[Calinou]

Note that implementing #3908 should improve directional shadow quality a fair bit already, without going through the added complexity of virtual shadow maps.