This commit introduce the computation of a depth pyramid containing min and max depth values of the original depth buffer.
This is useful for Clustered Light Culling but also for raytracing on the depth buffer (SSR).
It's also usefull to have to fetch higher mips in order to improve texture cache usage.
As of now, 1st mip (highest res) is half the resolution of the depth buffer, but everything is already done to be able to make a fullres copy of the depth buffer in the 1st mip instead of downsampling.
Also, the texture used is RG_32F which is a too much but enough to cover the 24bits of the depth buffer. Reducing the texture size would make things quite faster.
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