The padding on either edge of the screen that any part of the magnifier cannot exist past.
This includes any part of the magnifier, not just the center; for example, the left edge of the magnifier cannot be outside the horizontalScreenEdgePadding.v
If the screen has width w, then the magnifier is bound to
_kHorizontalScreenEdgePadding, w - _kHorizontalScreenEdgePadding.
final double horizontalScreenEdgePadding;