Append a quad to an annotation's /QuadPoints array
Source: R/annot_authoring.R
pdf_annot_append_quad.RdWraps FPDFAnnot_AppendAttachmentPoints. Each quad is four
(x, y) points giving the corners of a tile in
counterclockwise order (matching the shape
pdf_annot_quad_points() reads back). For highlight /
underline / squiggly / strikeout annotations a quad covers each
affected text run; a typical paragraph-spanning highlight has
one quad per visual line.