Mint support

[LPGhatguy]

In order to support all the math libraries, it would be useful if glsl-layout would support conversions from any type that can be converted into the corresponding mint type.

I put together something similar for the (flawed) shader-types crate, but with a derive macro I think the ergonomics can be even better.

[zakarumych]

Yes, this sounds useful. I would be glad to merge a PR with this :)