Re-enable Mixed particle rendering

There seems to have been some copy-pasta and maintenance issues that have caused Mixed rendering to be unfunctional.
This commit fixes those issues and re-enables mixed rendering, as explained [here](http://www.garagegames.com/community/blogs/view/22804).

Currently renders the low-res particles at 1/4 the resolution. To increase or decrease this, look at line 77 in RenderParticleMgr.

Performance difference with 2 emitters,  each emitting 1000 particles per second: 

| Method | MSPF | Impact |
| ------- | :-------: | :-------: |
| Highres | 27 | 8.5 |
| MixedRes | 20 | 1.5 |
| No emitters | 18.5   |