Add spiral emitter type

Adds the spiral emitter type w/ optimizations as previously discussed. An example of the updated effect can be seen here (other stuff in the scene can be ignored):

https://www.youtube.com/watch?v=r25zk_J_8mI

It is used here:

https://github.com/movitto/omega/blob/master/site/source/javascripts/omega/galaxy/gfx.js#L41

I passed the emitter itself to the vertex initializer call and reused the 'update existing' call to consolidate code. If you'd rather me reproduce the algorithm in both places or file this PR request against another branch I can easily do that. Shout out if there is anything else that could be tightened up further.