Bias toward nearest whole percentage

[andeston]

Summary

Fixes consistency issues for devices with low precision when adding or subtracting percentages.

Fixes #118

Behavior

Before

$ ./brightnessctl -m --frac s 50%
amdgpu_bl1,backlight,128,50.2%,255
$ ./brightnessctl -m --frac s 10%+
amdgpu_bl1,backlight,154,60.39%,255
$ ./brightnessctl -m --frac s 10%+
amdgpu_bl1,backlight,180,70.59%,255
$ ./brightnessctl -m --frac s 10%+
amdgpu_bl1,backlight,206,80.78%,255
$ ./brightnessctl -m --frac s 10%+
amdgpu_bl1,backlight,232,90.98%,255

After

$ ./brightnessctl -m --frac s 50%
amdgpu_bl1,backlight,128,50.2%,255
$ ./brightnessctl -m --frac s 10%+
amdgpu_bl1,backlight,153,60%,255
$ ./brightnessctl -m --frac s 10%+
amdgpu_bl1,backlight,179,70.2%,255
$ ./brightnessctl -m --frac s 10%+
amdgpu_bl1,backlight,204,80%,255
$ ./brightnessctl -m --frac s 10%+
amdgpu_bl1,backlight,230,90.2%,255