fix: smudging at some fractional `devicePixelRatio`

[WofWca]

Here's 1.1 DPR (page zoom):

As you can see, the lines appear to be of different width/color. It's especially noticeable for 1px-wide lines. The effect is much more visible in motion.

Probably need to look here:

https://github.com/joewalnes/smoothie/blob/cd7a6b543c11ebddb1d216f1c5d8f3f01ec3ce80/smoothie.js#L843-L845