updated blend mode when padding 0

[SandroMaglione]

Fixes #32

The issue occurred when padding: 0, gradient applied, and roundedEgde applied. It was causes by blendMode.

Example:

class GradientRoundedEdge extends StatelessWidget {
  const GradientRoundedEdge({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
          child: Padding(
        padding: const EdgeInsets.all(50.0),
        child: Column(
          children: [
            Container(
              color: Colors.grey[600],
              padding: const EdgeInsets.all(10),
              child: const StepProgressIndicator(
                totalSteps: 100,
                currentStep: 40,
                size: 30,
                padding: 0,
                roundedEdges: Radius.circular(100),
                selectedGradientColor: LinearGradient(
                  begin: Alignment.topLeft,
                  end: Alignment.bottomRight,
                  colors: [
                    Color(0XFFFFE2CD),
                    Color(0XFFFEC2E7),
                    Color(0XFFC9E7FF),
                    Color(0XFF86FEF4),
                  ],
                ),
                unselectedGradientColor: LinearGradient(
                  begin: Alignment.topLeft,
                  end: Alignment.bottomRight,
                  colors: [
                    Color(0XFF171C21),
                    Colors.black,
                  ],
                ), // Needed
              ),
            ),
          ],
        ),
      )),
    );
  }
}