This is added as part of the GLM_EXT_matrix_common extension, as this
function is not provided by the core GLSL specification (version 4.20).
The implementation of this glm::abs(mat<>) function mirrors the
glm::abs(vec<>) implementation. It should be functionning the same in
every way as the vec implementation.
Bonus points : AFAICT this allows to vectorize operations on compilers
that support optimization of these patterns, just like the functor1<>
struct in _vectorize.hpp for vectors.
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