Retrieve initial texel data for mips, starting from FirstMipToLoad, up to the last mip in the texture. This function will only return mips that are currently loaded in Bulk Data. THIS FUNCTION WILL FAIL if you include in the requested range a bulk data mip that is NOT currently loaded. Also note that only inline mips are loaded initially, and that ALL mip data buffers are discarded (either returned or freed). So... in short, this function should only ever be called once.