Simplify ImageColors::generatePalette initial loop to fix crash

tempSamples is meant to work like a pseudo thread local storage, before
pushing everything together.

On my system omp_get_thread_num always returned 0 despite running in
different threads. We then crash on multiple access to the same list.

Maybe that's an upstream bug, but in either case we can reduce some
allocations, complexity and copying by locking round the insertion.