No invert() for Mat3 and others

[faulesocke]

Currently it seems like invert() is only defined for Mat4 types, but not Mat3 and Mat2. Is there a reason for this?

[yoanlcq]

Hi, well not really, it's only because inverting Mat4 types is all I needed. I'm totally OK with implementing this though, or accepting PRs that add this.

In practice I often found Mat3 values to be orthogonal (when they are simple combinations of translations, rotations and scales), in which case the inverse can be obtained by transposing.

A workaround is also to convert the Mat3 to Mat4, then invert the Mat4, then convert back to Mat3, but I agree it's not great.