gl2: Make New return an interface instead.

[emidoots]

If you look at the docs for the gfx/gl2 package: https://godoc.org/azul3d.org/gfx/gl2.v2

Majority of the methods defined on gl2.Renderer are simply implementing the gfx.Renderer interface. It's hard to tell from the docs which ones are not part of gfx.Renderer -- a better solution would be to make gl2.Renderer a interface:

type Renderer interface {
    gfx.Renderer
    // Other OpenGL 2 specific renderer things.
}

[emidoots]

After:

Before: