set triangle tree subtree depth to 3 (however triangle tree layout optimization is disabled by default).
forced inlining of a couple methods.
added faster and more precise alternatives to the functions defined in foundation/math/fastmath.h (the old ones will be removed in the next commit).

these alternatives were borrowed from https://code.google.com/p/fastapprox/.

extensive performance and accuracy analyses were performed.
removed old functions from foundation/math/fastmath.h.
small tweaks in unit tests.
foundation::GaussianFilter2<T> now uses foundation::fast_exp() instead of std::exp().