ector: move gradient color computation to a pool of thread.

ector: move gradient color computation to a pool of thread.

This has been a long standing plan for improving performance in rendering
vector object. If your test involve updating gradient, you will get another
speedup of around 15%. Combined with previous shape, we get a 65% improvement
with doing the CPU intensive computation in there own thread before the
rendering kickoff. This was motly theorical until now, but well, it works
great !