Relative + and - are not inverses

[andeston]

Alternating relative addition and subtraction can result in the value drifting.

$ brightnessctl -m s 50%
amdgpu_bl1,backlight,128,50%,255
$ brightnessctl -m s 10%+
amdgpu_bl1,backlight,154,60%,255
$ brightnessctl -m s 10%-
amdgpu_bl1,backlight,129,51%,255
$ brightnessctl -m s 10%+
amdgpu_bl1,backlight,155,61%,255
$ brightnessctl -m s 10%-
amdgpu_bl1,backlight,130,51%,255
$ brightnessctl -m s 10%+
amdgpu_bl1,backlight,156,61%,255
$ brightnessctl -m s 10%-
amdgpu_bl1,backlight,131,51%,255
$ brightnessctl -m s 10%+
amdgpu_bl1,backlight,157,62%,255
$ brightnessctl -m s 10%-
amdgpu_bl1,backlight,132,52%,255
$ brightnessctl -m s 10%+
amdgpu_bl1,backlight,158,62%,255