Particles are go

I finally figured out what I was doing wrong with the particles. I'm still not happy with them, but I can see them now. I was using CustomVertex.PositionColored, since I was providing a position and a color for each particle. I wanted to use CustomVertex.TransformedColored, since I am providing screen-space coordinates for each particle. I'm guessing that the transforms from world to screen space were pushing the particles off the screen before. So, they exist, but they're ugly. I'll work on that next.