rename gl to gfx_gl

This is done to avoid this warning:

```
warning: using multiple versions of crate `gl`
main.rs:25:1: 25:40 note: used here
main.rs:25 extern crate snowmew  = "snowmew-core";
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: crate name: gl
main.rs:25:1: 25:40 note: used here
main.rs:25 extern crate snowmew  = "snowmew-core";
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```