PC3: Improve support for RTL sliders
Fixes steps ticks not highlighted under current value in RTL. Fixes slider groove visibly collapsing in front of a handle near the start of a slider, i.e. close to the `from` value by extending the groove up to at least a half of the handle, but still filling 100% space when `value === to`. Removes duplicated (identical) code for an extra tick: it works just as fine with an incremented numeric model. Rounds up all potentially floating sizes to ensure that there are no visual glitches or off-by-one-pixel jumps back and forth.