DrawList: fixed baked corners to better match polyline corners

- added CalcArcStepAndCount() to calculate the step size consistently
- _AddRectTinyRounding: fixed stem placement with thin lines (clamping was needed for an older version of the function)
- _AddRectTinyRounding: fixed texture coordinates (uses if inner/outer were reversed)
- use polygon instead of circle when calculating baked corner rounds (the circle was more accurate but looked different size comparet to polyline)