1.12 performance low-hanging fruit

I've been doing a little profiling, because I've been seeing more buffer underruns than normal.  I identified the following issues:

* vinyl spinny quality drawing was super slow, and the work was being done even if vinyl wasn't active or drawing was off.  I fixed this during the spinny work.
* EngineFilterDelay::process takes lots of CPU for some reason, I don't know why.  It's often the top item in the profile.
* EngineBuffer::isActive is super slow.  Apparently checking a QSharedPointer for non-null is slow3.  Adding a bool that tracks the null/notnull status of the pointer solves the problem, but we have to think about possible race conditions.
* EngineVinylSoundEmu is slow.  It should noop if all of the conditions are false (!wasDithering && !isDithering && !prevQuieting && !curQuieting)

Of these, I have fixes for all of them except the Delay issue.