Feature request: Particle gravity wells

[AndrewRayCode]

After looking at the first video on http://gamedevelopment.tutsplus.com/tutorials/make-a-neon-vector-shooter-in-xna-particle-effects--gamedev-10111 - particle gravity emitters with definable positions, strengths, and radiuses of influence would be AWESOME.

So would the ability to define simple collision planes (but that may be a bit too much)

[squarefeet]

I'm all for this (I did start to experiment with gravity wells - see the gravity branch - that was the reason you saw the dev branch doing weird things recently), but we need to be mindful of quite a few things:

• Performance: I don't want to just add features without giving due thought and care to any possible performance impacts the feature could have. I want to keep this library as efficient as possible - that's the reason for its existence in the first place.
• Number of shader attributes: Not all GFX cards / devices can support the same number of attributes for a shader. We're already bumping dangerously close to what I consider to be the upper limit of 16 (see here)

Like I said, I'm well up for this, but we will have to work thru the issues above first!