[rbp] Make gui limit default to 720 when memory is limited

On a 256M Pi, it's safest to default to 720p gui as memory is quite limited.
I have seen people reporting using other values (as some compromise between memory usage/performance/quality).
But you might be right that a short fixed list may be better.
I think forcing 720p (even on 512M Pi) and forcing unlimited/1080p (on a 256M Pi where add-ons and network streaming are not used) are valid options in addition to auto setting.