Force texture completion for copy. See gl-320-fbo-multisample-explicit

* When we do glCopyImageSubData, this seems to require a complete texture
  across all mips - ie. the full mip chain (up to MAX_LEVEL) must be set.
  This requirement holds even if we only copy the mips that are present.
* However, for example attaching an image to a framebuffer, it doesn't
  have to be mipmap complete even if the min/mag filters want mipmaps.
  So in this case we have to force the texture to be complete enough for
  our purposes, and we do this just by setting MAX_LEVEL to clamp the mips