mipmap support on DDS for openGL cubemap
glGenerateMipmap on uncompressed cubemap
OpenGL: Mipmaps for GFXGLCubemap.
OpenGL: Mipmaps for GFXGLCubemap. Fix mipmaps size.
OpenGL: Mipmaps for GFXGLCubemap. Fix compressed textures.
OpenGL: Mipmaps for GFXGLCubemap. Fix dynamic cubemaps.
Merge pull request #1056 from Lopuska/patch-12

mipmap support on OpenGL cubemap