Fix #856: Polyline border drawing

[Ahmed-gubara]

Fix for Issue #856
This fix Polyline border drawing bug introduced in version 0.11.0 PR #766 .
Causing the bug was that borderPaint.style = PaintingStyle.stroke was set after the drawing line _paintLine(...,borderPaint), which made the drawing use the default PaintingStyle.fill causing the border to create a filled shape.

flutter_map/lib/src/layer/polyline_layer.dart

Lines 188 to 195 in e8389e1

	if (borderPaint != null) {
	if (filterPaint != null) {
	filterPaint.style = PaintingStyle.stroke;
	_paintLine(canvas, polylineOpt.offsets, borderPaint);
	}
	borderPaint?.style = PaintingStyle.stroke;
	_paintLine(canvas, polylineOpt.offsets, filterPaint);
	}

Reordering the style set statement before the _paintLine(...) statement fixed the bug.

[Ahmed-gubara]

Hey @kengu, can you please review the changes?

[kengu]

LGTM

[kengu]

@Ahmed-gubara Tested locally and it seems to work as expected. Thanks for the contribution!