patch 9.2.0872: popup with opacity does not use the font of the highlight group

patch 9.2.0872: popup with opacity does not use the font of the highlight group

Problem:  When a popup window has an opacity between 0 and 99 the "font"
          item of its highlight group is ignored and the regular font is
          used instead.  The same applies to "ctermfont", and to the
          popup menu when the "opacity:" value of 'pumopt' is used
          (Salman Halim).
Solution: Also take the font from the popup highlight when blending the
          cell attributes (Hirohito Higashi)

fixes:  #20551
closes: #20866

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>