Particles rendered in front of laser bitmaps

@AxemP reported this on IRC.

Opaque particle effects obscure laser bitmaps even though those bitmaps are located in front of them in the world. This is caused by how the particle shader uses the position or depth buffer for determining the soft-particle intensity. Since laser bitmaps are not rendered into either of those buffers, the particle shader doesn't know that there is a bitmap there and renders the particle bitmap on top of the laser bitmap.

Test mod provided by @AxemP: https://drive.google.com/open?id=0B_q0tkX7VcSaZXZHQ0w4bDZsLTg