[Stk] fast time domain filter implementation

[Emile Vrijdags]

In response to Steven and Immanuel

The problem is that the profiler (vtune) requires the project to be 
built in debug mode (afaik: I've been using it since last week), and no 
optimizations are present. So I probably have a distorted image of the 
performance of that function.

I  tried out some extra (all i could find) optimization flags, (standard 
it was configured with the /O2 flag) and got about 30 to 40% more 
performance with all optimizations on, when timing a filtering loop with 
clock() in time.h (probably not ideal).
Also I noticed that the intel compiler has lots more optimization flags 
than the vs2005 compiler, and with the same optimizations the intel 
compiler seems still a little faster. I used: /O3, /Og, /Op, /GA, /Qfp, 
/Qaxt, /Qxt, /Qparallel

In my full application using opengl in multithreaded environment (opengl 
renderloop, an update filterparameters loop, and the stk callback loop) 
there is an even more significant performance gain between vs2005 and 
intel compilers. I can use about double the amount of filters before the 
audio starts to be chopped up.