* ThreadableJob: Move from AtomicInt to std::atomic
This is the first in a series of commits that migrates away from
AtomicInt towards the C++11 standard library types.
This would allow the removal of AtomicInt and related Qt
version-specific code.
While we're at it, also make ProcessingState an `enum class` so that
it's not implicitly convertible to integers.
* LocklessAllocator: Switch from AtomicInt to std::atomic_int
If it looks like some assignments around the Compare & Swap loops went
missing, it's because compare_exchange_weak() updates the first argument
to the current value when it fails (i.e., returns false).
* BufferManager: Remove extra AtomicInt include
* MixerWorkerThread: AtomicInt to std::atomic_int
* NotePlayHandle: AtomicInt to std::atomic_int
* Remove AtomicInt.h
* Move from QAtomicPointer to std::atomic<T*>
This removes some #ifdef trickery that was being used to get
load-acquire and store-release from Qt5 and "plain" (presumably
sequentially-consistent) loads and stores from Qt4.
