Adding LightningStrike, LightningStorm with examples

[yomboprime]

Support to create lightning strikes and other voltaic arcs, animated in time.

These are 3 examples in one html, and two new classes:
THREE.LightningStrike is a BufferGeometry which varies over time (a call to .update( time ) is needed) Rays can have a limited lifespan or can be 'ethernal'.
THREE.LightningStorm is a higher class object, easy to use and drop-in as a storm in a game. It creates rays on its own over time. (Also needs .update())

The two classes are created with parameter objects (details in the file headers). You can define some 'callbacks' to control the form of the rays, do something when a ray hits the ground, and other things.

Edit: Also the rays have a read-only state, so you know each phase of the life cycle including propagation and vanishing.

Time can go forwards and backwards, be slowmotion, etc.

I had the need to do this for some time. 😊

[looeee]

For testing: https://raw.githack.com/yomboprime/three.js/storm/examples/webgl_lightningstrike.html

Working nicely on my Moto G6 👍

[yomboprime]

Oh yes, I forgot the live link:
https://yomboprime.github.io/lightning_strike_demo/webgl_lightningstrike.html

[jsantell]

Heh, pretty cool 😄

[mrdoob]

Thanks!