Scalar multiplication

[Jaxan]

One of the annoying parts of glsl is that the following doesn't compile:

glm::vec3 v(1, 2, 3);
glm::vec3 w = 2 * v; // error :(

You correclty mirrored this in glm, which is good. But in our old codebase we did these kind of multiplications, because they are just too convenient ;-). Instead of changing our code, I added these multiplications.

[Groovounet]

I need to make a small update for C++98 support but that look fine.

Thanks for contributing!
Christophe