[Improve] Wrap CustomPaint in ClipRect to respect the boundaries

[rodolfogoulart]

Just wrap you CustomPaint in a ClipRect, so when displayed on some widget dont go beyond the boundaries.

In my case, using the Stack with the property fit = StackFit.expand has a good result too in the screen

@override
  Widget build(BuildContext context) => Stack(
        fit: StackFit.expand,
        children: [
          widget.child,
          IgnorePointer(
            child: ClipRect(
              child: CustomPaint(
                foregroundPainter: _ArrowPainter(
                  _elements,
                  Directionality.of(context),
                  [this, ...widget.listenables],
                ),
                child: Container(),
              ),
            ),
          ),
        ],
      );

Example

[Schwusch]

Hey, thanks for the input! Can I ask you for a pull request with the changes you want?