recreated dist/
Only calculate the value from the position, if the it is well defined.
Otherwise leav the value alone.
If the slider is initially invisible (display: none), the size of the
slider might be 0. But it may have a well defined value.
This change preserves the value.
Don't call the onSlide callback during initialisation and size updates.
Add a third argument initialValue to the callbacks onSlide and
onSlideEnd.
It is thr value of the slider at start of the current user interaction.
If the callback is invoked due to a programmatically change of value,
initialValue is undefined.