Problems with gl.Texture ang gl.GLenum

Problems with texture and glenum

I have updated the repo to use:
github.com/go-gl/gl/v2.1/gl

But using it version or other, i always got:

```
engine/Texture.go:109: undefined: gl.Texture
engine/Texture.go:112: undefined: gl.GLenum
engine/Texture.go:113: undefined: gl.GLenum
engine/Texture.go:115: undefined: gl.GLenum
```

What i need change to make it work and make a pull-request?
