patch 9.2.0520: Reversed text opacity in popup when termguicolor is set

patch 9.2.0520: Reversed text opacity in popup when termguicolor is set

Problem:  When termguicolor is set, popup opacity seems reversed
          for the underlying text: when opacity go from 1 to 99,
          the greater opacity is, the more underlying text is readable.
Solution: Invert popup_color and base_fg when calling blend_colors
          (Shad).

fixes:  #20283
closes: #20284

Signed-off-by: Shad <shadow.walker@free.fr>
Signed-off-by: Christian Brabandt <cb@256bit.org>