tapScaffoldEnabled -> _invisibleCover() dar

[noobloser]

While the parameter tapScaffoldEnabled works as intended, it would be great to gain access over the darkening of the main scaffold.
Eg. for tapScaffoldEnabled = true it is turned off, for false it is turned on by default.

Additional shameless Feature Request

An additional cherry on top would be the possibility to add a clipper for a ClipPath above the Container of _invisibleCover(), so certain elements can be excluded from the darkening.