The geometry the pool just acquired for this commit.
The mesh's geometry before the acquire.
prevGeometry.instanceCount read BEFORE the acquire: a pool grow releases
the old geometry without ending its held draw, so this is the count that
was actually on screen.
The element count the previous commit recorded.
This commit's (capacity-clamped) element count.
getPrefixParent of this commit's data (read before it is consumed).
getCommittedData of the mesh: what its last commit uploaded.
The GPU still holds the committed prefix exactly as this commit would
write it (gpuPrefixIntact plus each type's parity conjuncts), so the
texel upload may be suffix-only.
The pre-commit facts planInstancedOrdering decides from.