Adding an attribute to skip the margin offset for the fab

[ericmartineau]

In my case, I was positioning the FAB using a stack (I'm using ios scaffold widgets).

The margin offsets were causing my button to float slightly to the left and downward. This attribute allows for removing the margin offset transformation.

[marianocordoba]

Hi @ericmartineau. Thank you for your PR! Can you please update the README with the documentation for the new property?

[MrCsabaToth]

It'd be so great if we could just ditch the

  Container(
      margin: widget.fabMargin,
      // Removes the default FAB margin
      transform: Matrix4.translationValues(
        16.0 * _directionX,
        16.0 * _directionY,
        0.0,
      ),
      child:

and just start the build return with the Stack. It works for me,