[videoinfoscanner] re-use fasthash if possible

No need to compute a slow hash in case the fast one already succeeded. Same already happens for shows.

@jmarshallnz for review please.
The comment at ln 306 is now wrong, right?  They (hash, dbHash) can't be the same and get into this block: If they're the same, bSkip has been set.

I *think* that you can remove the hash != dbHash in the if condition at ln 298, and remove the && !dbHash.empty() in the condition at ln 307.

Still, IMO with this bSkip being set in a bunch of spots, it's still not as obvious at it should be how the code flows.  e.g. the first if (!bSkip) mayaswell be an else.  Similarly, there's no clear block that determines the logging/state, which is fully determined based on dbHash and hash.

Lastly, you've changed behaviour here in that bSkip is not set to true in the "skip due to empty/not existing, add to clean list".