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, or available via derived data. THIS FUNCTION WILL FAIL if you include in the requested range a streaming mip that is NOT currently loaded. Also note that ALL mip bulk data buffers are discarded (either returned or freed). So... in short, this function should only ever be called once.