Fix missing mut in Mat4::as_mut_array

[sjb3d]

Hello again, this is a typo fix to make the output of Mat4::as_mut_array mutatable, to match the other matrix sizes.

If you have time to take a look for the next update then that would be awesome, thanks!

[fu5ha]

Sure, I'll do one now :) This is a breaking change of the api, buuuuuut the fact hat it was clearly wrong and you're the first person to report it suggests to me nobody else has used it, so I think I'll cheat and get it into a patch version anyway

[sjb3d]

Great, thanks!