top/xbeam/persist: unify decay/persist, reduce flickering

[vk2seb]

• Unify beam 'decay/persist' options so we always travel the framebuffer at the same speed (even for slow decays), but randomly skip pixels using an LFSR. This makes flickering almost invisible at slow decays.
• This is needed as we only have 4 bits for intensity, so we can't reduce by 'fractional bits' without increasing memory bandwidth.