auto-texture: revert _new_with_size semantics

This reverts the change in semantics for cogl_texture_new_with_size so
that it goes back to allocating textures synchronously and returning
NULL if there was a failure to allocate. Only the new/2.0 texture apis
will have the lazy allocation behaviour so we avoid breaking existing
code. This also fixes a potential crasher by removing a code path
that was passing NULL to cogl_texture_allocate() that would have caused
and abort if there were an error.

Reviewed-by: Neil Roberts <neil@linux.intel.com>