Fixed goofy fluid rendering, affecting the Smeltery GUI.

I could probably have avoided those 4 new vars, but I've messed a lot with code until I got to that solution, and ended up not cleaning the vars up. I'm the kind of person who'd rather store a value in a var and use it than call a getter twice. In the case of the interpolations it's not a simple getter, as there's a bit f math involved.

before:
![before](http://puu.sh/9AJx1/4a9e3ea4cb.png)

after:
![after](http://puu.sh/9AJEM/653214a341.png)

These are both images of the same smeltery with the same amount of molten platinum from TE3,
All that changed was the GUI code.