[minor feature] Add macros usable as initializers

[pm4gh]

In vec3.h there are pairs of macros like GLM_VEC3_ONE_INIT and GLM_VEC3_ONE.

Would it be possible to add similar _INIT ones for GLM_XUP, GLM_YUP and GLM_ZUP ?

[recp]

Hi @pm4gh,

Yes, definitely. Added to TODOs.

👍