System.Drawing: Graphics.DrawRoundedRectangle / FillRoundedRectangle produce anti-aliased corner artifacts

[KlausLoeffelmann]

Problem

Graphics.DrawRoundedRectangle and Graphics.FillRoundedRectangle (System.Drawing.Common,
gated under NET9_0_OR_GREATER) currently build an arc-based GraphicsPath and stroke/fill it
with the default PixelOffsetMode and no anti-aliasing setup:

public void DrawRoundedRectangle(Pen pen, RectangleF rect, SizeF radius)
{
    using GraphicsPath path = new();
    path.AddRoundedRectangle(rect, radius);
    DrawPath(pen, path);
}

Because the corner arcs are anti-aliased on a different sub-pixel grid than the crisp straight
edges, the rounded corners render with visible artifacts: the arcs look softer / offset relative
to the straight sides, and a centered stroke can be clipped when the rounded rectangle fills its
bounds.

Expected

Rounded-rectangle corners should blend seamlessly with the straight edges and produce a crisp,
uniform border and fill.

Repro

Draw a rounded rectangle with a modest corner radius and a border a few pixels thick and observe
the corner arcs versus the straight edges — the arcs appear blurrier / shifted.

Proposed fix

Apply a "high-quality inset stroke" technique inside the two methods:

• Set SmoothingMode.AntiAlias and PixelOffsetMode.HighQuality (saved and restored around the
  call so the caller's Graphics state is not permanently changed).
• For the stroke, inset the geometry by half the pen width so the entire border sits inside the
  requested bounds on a consistent sub-pixel grid.
• Fill uses anti-aliasing + high-quality pixel offset (no inset needed).

This realigns the arc anti-aliasing with the straight edges and removes the artifact, while
keeping the public API signatures unchanged.