Update indicator padding calc

[hysuki]

Indicator padding issue, Indicator size gets smaller as it gets closer to the edge.

Example

indicatorStyle: IndicatorStyle(
        width: 30,
        height: 30,
        indicatorY: 0~1.0
...

indicatorY	image
0
0.5
1

[JHBitencourt]

Thanks for the PR.
I'll give it a look soon.

[JHBitencourt]

I appreciate your PR, but I'm currently not accepting this one.

It does not address and fix all the situations when calculating the indicator padding.
In order to support horizontal timelines I refactored this calculation, as you can see here: #16

Thanks! :D