Tweaked colour generation in the emitter.
Added RotateToFaceDirectionModifier, a modifier that will rotate a particle so
it faces its direction of travel. Good for sparks.
Hacked around a bug in the AgeModifier. Time is updated at a different rate in
the emitter than in the modifiers, because a modifier is not necessarily updated
each time emitter.Update is called. This difference can lead to Age being less
than zero or greater than 1, which breaks some things.
Fixed OpacityFastFadeModifier so that it can work with premultiplied alpha
by storing the initial colour.
Particle velocity is now also copied to the memory buffer. Particle size
increased to 44 bytes for the extra two floats.