PdfSelectionHandlePainter

Draws one selection boundary marker ("thumb") for PdfView.

Called once per edge inside the page's draw pass. x is the boundary's horizontal position; top/bottom are the boundary line's vertical extent, all in page-slot pixels. Size the marker against bottom - top (the line height) so it scales with the text through thumbnails and deep zoom, like PdfSelectionHandleDefaults.CaretAndDot does.