Initialize data prelimary to theGetMips()step. Can be called several time (it does not have to advance to GetMips immediately). Mostly useful to simplify the logic inGetMips(). This is because GetMips is a chained call between all mip data providers, each taking some mips to handle, and is not compatible with multi step process. This means thatGetMips()must return immediately and can not postpone or delay return by not advancing to the next steps.