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

I don't see how to do that without branching. If no branching is a requirement for fastCos, then I should put this code somewhere else, like gtx/determinist_trigonometry.hpp?