More behavior control for particles:
- ParticleSystemSceneNode does now affect the particle movement direction by default. Can be disabled to get old behavior.
- Allow interpolating postions which is useful when particles are attached to fast-moving nodes (to avoid regular patterns).
- Invisible behavior flags from check-in yesterday re-implemented with new general behavior flags.


git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4770 dfc29bdd-3216-0410-991c-e03cc46cb475