Fix the afterimage texts that appear when the number of processes changes

[XuehaiPan]

When a new process is created or an old process is gone, command python3 -m gpustat -i 5 --show-all --show-full-cmd does not remove the old texts on the terminal.

[Stonesjtu]

Great. Is it possible to add a test case for this bug fix?

[XuehaiPan]

Is it possible to add a test case for this bug fix?

This issue only occurs in watch mode (with --watch / -i / --interval option). And test cases in test_gpustat.py are testing for normal mode (without stdout redirection to blessed.Terminal()).

[wookayin]

Thanks, I will have a look soon.

[wookayin]

Works well perfectly as expected and explained, thank you very much @XuehaiPan.