Remove polygon rendering hacks

[xTibor]

egui currently doesn't support rendering concave polygons, so egui_extras_xt have to use the convex polygon rendering methods with some hacks to render concave polygons. Remove these workarounds when egui gets support for rendering concave polygons.

emilk/egui#513

Affected places and hacks:

• WidgetShape::paint_shape: manual tessalation with two-stage rendering
• WidgetShape::paint_arc: manual tessalation with two-stage rendering, additional special case for misrendered degenerate polygons
• CompassMarkerShape::Star: no hacks, broken polygons are barely visible at small sizes
• SegmentedDisplayWidget::ui: manually reordered segment vertices to avoid visible artifacts