Implement Default trait for vectors

[yuraiz]

I made a gtk widget that uses graphene::Vec3 internally,
https://github.com/yuraiz/telegrand/blob/animated-background/src/session/content/gradient_bg.rs
and because vectors doesn't implement Default I can't derive Default for widget implementation

also it would be useful to have gdk::RGBA to vector conversions but that's compicated because those libraries are independent

[sdroege]

Probably makes sense to also implement that for the Points the same way, and for the Matrix as the identity matrix.

Do you want to add those?

[yuraiz]

Probably makes sense to also implement that for the Points the same way, and for the Matrix as the identity matrix.

Do you want to add those?

Yes

[yuraiz]

I found that vectors have zero() fn, it's better to use it