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For 8-bit the subtrahend is small enough to fit into uint32_t.

This is the same that was done for:
c0241664 Resolve -Wshorten-64-to-32 in variance.

For 10/12-bit apply:
63a37d16 Prevent negative variance

Change-Id: Iab35e3f3f269035e17c711bd6cc01272c3137e1d