Resolve -Wshorten-64-to-32 in highbd obmc/ext-inter variance.

For 8-bit the subtrahend is small enough to fit into uint32_t.
Prevent negative variance in 10/12-bit.
Previously:
51e0c54ed Resolve -Wshorten-64-to-32 in highbd variance.

BUG=aomedia:445

Change-Id: Ibb9bfc5e926de354bbab80127012e5e1861348a3
2 files changed