fractional scaling: rounding error

[MaxVerevkin]

Steps to reproduce:

• Set scaling to 1.25.
• Create a small floating view which supports fractional scaling (I will use alacritty, from my testing it correctly follows the protocol).
• Move it around in steps of one pixel (riverctl move left 1).
• Observe that sometimes the contents of the view are blurred and sometimes not. This looks like a texture is stretched or compressed by one pixel.

Note: this does not only happen to small and floating views (large tiled are affected too), but I find this to be more reproducible.