Only reupload all cubemap faces for the first glTexImage2D call

* We need a workaround for cubemaps where our redundant-glTexImage2D call has
  dropped a glTexImage2D on level 0 of another cubemap face that should be
  'initialising' it. What we do is just upload all faces. Initial contents will
  then replace the contents.
* However, this assumes there will be initial contents to replace. If we only
  have a set of cubemap uploads because the cubemap was uploaded within the
  captured frame, we can't do this replicate each time or the last face uploaded
  will be splatted across all. Instead we check to see if this is the first
  glTexImage2D call on a level, and only replicate in that case.