FontRun.UpdateOverhang uses wrong offset for glyph pointer

[Beldaa]

In FontRun.UpdateOverhang() (FontRun.cs line 521), the glyph pointer passed to GetGlyphWidths uses Start (the code point index in the text block) instead of Glyphs.Start (the offset into the underlying glyph array).

// Current (wrong):
paint.GetGlyphWidths((IntPtr)(pGlyphs + Start), sizeof(ushort) * Glyphs.Length, out var bounds);

// Should be:
paint.GetGlyphWidths((IntPtr)(pGlyphs + Glyphs.Start), sizeof(ushort) * Glyphs.Length, out var bounds);

Start is the code point index of this font run in the overall text, while Glyphs.Start is the actual offset into the glyph buffer. These differ whenever glyph count != code point count (ligatures, combining marks, surrogate pairs, complex scripts).

This causes incorrect overhang/bounds calculations, affecting MeasuredOverhang results particularly with non-Latin text or fonts with ligatures.