Sub-frame emitters with interpolated trajectories

[malleusinferni]

Currently, particle spawn rate is decoupled from render framerate in a way that works for static emitters. For moving emitters, each particle's initial position is still framerate-dependent. An emitter that moves fast enough will leave visible gaps between the batches of particles it spawns on each frame. If the render pipeline stalls, these gaps can grow much larger.

Reducing this framerate dependence requires providing information about an emitter's trajectory to the particles it spawns, so that a batch of particles can be distributed amongst interpolated points along this trajectory. For the time being, it should suffice to linearly interpolate between the previous and current GlobalTransforms of the emitter entity. The same interpolation method can be applied to other parameters as well.