Fixed PositionedTapDetector2's transformation of global to local coords

[bramp]

Now, before we accept this PR, there are two possible solutions

1.  // Transform the global coordinate to the local coordinates.
    final renderBox = context.findRenderObject() as RenderBox?;
    final relative = renderBox?.globalToLocal(global);

or

2. final relative = details.localPosition;

Option 2 is clearly the simplest, but it is unclear to me why that wasn't always the case. Perhaps localPosition wasn't a member when this code was written. (Which btw localPosition was added to flutter ~4 years ago). @TesteurManiak last touched this code ~6 months ago, and perhaps knows why local wasn't used.

[JaffaKetchup]

I can't think of a reason why we wouldn't use the second option.

I wonder if this will also open up the route to perspective with Transform working properly (to implement tilting)...?

[bramp]

From my understanding of localPosition it effectively does the same as my option 1. Also Option 1 will correctly support any transformation, skew,scale,translate,rotate, etc.

I'll amend this to just do option 2. It's less code and simpler.

[JaffaKetchup]

LGTM, many thanks once again!