Fade Out Behavior
This behavior changes a particle's alpha value to gradually fade out over its lifetime. This behavior can also be achieved using a color gradient behavior, however, the fade out behavior is easier to set up and more efficient at runtime.
StartAlpha: The alpha value to begin with when the particle has just spawned.
Exponent: How quickly to fade the alpha value from StartAlpha towards 0 over the particle's lifespan. An exponent of 1 results in a linear fade. An exponent of 2 will make it fade out much earlier, a value of 0.5 will make it fade out very slowly at first and then quite abruptly at the end.