More precision for fastCos and fastSin 

I tested the precision of glm::fastCos and glm::fastSin against glm::cos and glm::sin on the range [-pi pi]. The max error is ~0.2 on my computer.
I've made some changes and now it's ~0.000007. It's also slightly faster and work on a wider range.

To give some context, I had a problem of cross platform float determinism whith glm::cos and glm::sin, because theses methods rely on <cmath>.
Using the fastCos and fastSin solve my determinism problem, but the accuracy was not good enough anymore.

If you are ok with the code, I can improve the rest of the fast_trigonometry.
For reference, I used the same method as the one found in "A Guide to Approximations" by Jack G. Ganssle

That's branching approach concerns me too here, especially considering that fastCos goal is to be fast.