Func conveyor

It was used in Half-Life for the conveyor belts which objects could ride on. The surface material should have a  material proxy to show the movement even if nothing is on it, though this effect no longer looks very convincing and it's best to use nodraw on this brush.

SetSpeed fix
A small error was made while porting  which makes SetSpeed not aware that it should be taking a parameter. This line of code should be in bmodels.cpp, line 290: Change  to. It should work fine now.