Make Vec3A::from_vec4 a const fn

[herabit]

All of the implementations of Vec3A::from_vec4 do not seem to require non-const functionality, so marking them all as const fn will increase their versatility for API consumers.

[bitshifter]

Hi, yep that sounds good, thanks!

In case you aren't familiar with glam internals, you need to make the change in codegen/templates/vec.rs.tera and then cargo run -p codegen to regenerate the vec4 sources.

[bitshifter]

Ah it turns out this is not currently possible because Vec3::from_vec4 invokes deref to access the x,y & z values which can't be done in a const context.