[osg-submissions] Fix hardware mipmap generation for cube maps

[Farshid Lashkari]

Hi Robert,

I was getting a black image when enabling hardware mipmap generation on
cubemaps. I believe the problem is that the base Texture class is hard
coding GL_TEXTURE_2D when enabling mipmap generation, instead of using the
target type from the derived class. I've attached the small change that
fixed the issue for me.

Cheers,
Farshid