[Feature] Emitter replaces oldest/random particle when full

[djeedai]

Describe the solution you'd like

Split from #118.

When the particle effect is full (all particles spawned), the emitter kills the random / the oldest particle to allow spawning a new one, to ensure the spawning rate is truly constant and not clamped by the capacity of the GPU buffer.

Describe why you want that solution. Is this related to a problem?

Clamping to the capacity means no particle spawn until one die to make space for it, which will likely break the effect visually. This pushes effect authors to over-allocate the effect to ensure clamping doesn't occur. Replacing a random or the oldest particle often has very little consequence visually.

Describe alternatives you've considered

Over-allocating the effect. This wastes GPU resources.