cogl-texture-2d-sliced: Don't create the slice textures twice

cogl-texture-2d-sliced: Don't create the slice textures twice

Both of the cogl_texture_2d_sliced_new functions called the
slices_create function which creates the underlying GL
textures. However this was also called by init_base so the textures
would end up being created twice. This would make it leak the GL
textures and the arrays which point to them.