Add support for doing an identity initialization in the default constructor of fmat4x4SIMD().
Add support for doing an identity initialization in the default constructor of fmat4x4SIMD().

This is disabled by default and enabled with GLM_SIMD_ENABLE_DEFAULT_INIT. It makes toggling between SIMD and non-SIMD builds a bit easier.

novaquark/glm,ramassin/glm,hrehfeld/glm,ramassin/glm,novaquark/glm,dustin-biser/glm,hrehfeld/glm,rleigh-dundee/glm,nikodil/glm,nikodil/glm,dustin-biser/glm,rleigh-dundee/glm