Optimize QQuickItemParticle::prepareNextFrame().

This moves one hash lookup out of the inner loop, and most importantly
avoids detaching both the particle group hash and the particle data vector
on every iteration of the inner loop.

Change-Id: If9a127b347367ac9dc1e8ee738ff914784adb896
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>