The VSM variant bit was overloaded, it meant two different things
depending on the DEP bit (depth).
For standard variants (DEP = 0), it decides the type of the shadow
sampler used (PCF or 2D).
For depth variants (DEP = 1), it decides what is written during the
shadow pass (nothing, i.e. depth only, or EVSM depth moments).
We now clearly separate the two bits throughout the code.
This change should be purely source-cosmetic, there shouldn't be any
behavior changes.
Co-authored-by: Powei Feng <powei@google.com>
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