Move thread-pool down into comphelper for re-use elsewhere.

Change-Id: Ib27b8b1ccc07ff194035d6c2ef3d45c429e3cea1
thread-pool: re-work termination semantics to avoid problems.

We want a pre-spun-up, shared thread-pool that doesn't get its
workers created & joined frequently.

Change-Id: I29081e3a3e3849ca30e63fd080ee3315d99cbe8d
Switch image scaling to use the shared thread pool.

Change-Id: I7450e10b329247d972ef048f61b8ce1ca943aca8