This feature is controled by a feature flag and is turned off at this point.
This CL shouldn't change the existing behavior of shadowmap allocation as
long as the atlas feature is not enanled.
When enabled, shadowmap allocations are not limited to layers of the
shadowmap texture, instead, an atlas is used meaning a layer can
be shared by multiple shadowmaps. At the very least this can save
a lot of memory as smaller shadowmaps can be packed together.
Currently this feature breaks some VSM features:
- mipmapping
- blurring
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