Optimised overlay render logic - #30

Currently the IntroStepBuilder renders overlay on the left side even when there is more space on the right, because the getOverlayPosition function inside _StepWidgetBuilder verifies if there is more space on the sides compared to top and bottom spaces but it doesn't exactly calculate which side has more space.

Before:

After:

to reproduce the issue add padding to the right above centre widget here in the example

Like this:

      appBar: AppBar(),
      body: Padding(
        padding: const EdgeInsets.only(right: 100),
        child: Center(
          child: Column(