Use correct fractional scaling values

[leolost2605]

Mutter provides us with supported fractional scaling values for a monitor. We should use those instead of hard coding some values.
That's because good values can differ from the rounded percentage (e.g. 1.4935... instead of 1.5) or sometimes applying can fail because the value isn't support (e.g. 0.75 most (all?) of the time).