[Math] Fix SMatrix test18 on fst builds
[Math] Fix SMatrix test18 on fst builds

this test failed because of the different treatment of floating point numbers.
After an inversion of a matrix, the elements are checked to deviate of an epsilon from
a certain quantity. Epsilon is ~1.776-e14.
The error was:
```
inv result : Failure 1 different than 1
Comparison failed for Id(0,0) == 1 != 1. with delta == 2.26485e-14 > 1.77636e-14
```
This epsilon has been changed for builds with the fast-math flag enabled to ~2.8e-14.

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