Fix LOD calculation in D3D12DynamicLOD sample

[ArtemVetik]

In the previous implementation of ComputeLOD, the returned value could never reach LODCount - 1 because size is never zero.
As a result, the highest LOD was never used.

This change adds round() to the calculation to round to the nearest integer, ensuring that for very small size values the maximum LOD is correctly returned.

[GameDreamByHuo]

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