[Bug] deltaBottom is not calculated correctly in horizontal view

[AlexanderFarkas]

Element is SizedBox(width: 20, height: 60)
InviewList is horizontal (Axis.horizontal)

In 'isInViewPortCondition' I still receive such calculations:
final width = deltaTop - deltaBottom. (60)
But it should be 20.

deltaTop is calculated correctly, but deltaBottom is not, because it takes item's height, not width.

[AlexanderFarkas]

@rvamsikrishna Wrong part is here
It should be something like 'orientation == Axis.vertical ? size.height : size.width'

[AlexanderFarkas]

#37 @rvamsikrishna Fixed here.
Please, review and merge