getnative(): Add an `ex_thr` parameter to adjust the threshold for excluding little difference in calculation

[Nuevo009]

With the default threshold, sometimes we encounter awkward situations. Like

Manually setting it to 0.005 will get a better result

I have no idea about the naming of this parameter, if you have a proper name please let me know.

[Nuevo009]

In addition, changing this value may affect the result of the kernel evaluation, especially for high taps Lanczos.

[WolframRhodium]

Thanks!