patch 9.2.0332: popup: still opacity rendering issues

patch 9.2.0332: popup: still opacity rendering issues

Problem:  popup: still opacity rendering issues
Solution: Fix remaining issues, see below
          (Yasuhiro Matsumoto).

This PR fixes the following issues:

- Padding blend hole at wide char boundary: when a padding cell overlaps
  the second half of a wide character, the right half's attr value is
  unreliable. Use the left half's saved attr for blending instead.

- Wide char background split at popup boundary: when a wide character in
  an upper popup straddles the edge of a lower opacity popup, both
  halves got different background colors. Since terminals cannot render
  different left/right background colors for a wide character, detect
  the lower popup with popup_is_over_opacity() and use the non-popup
  side's underlying attr for both halves.

- Wrong blend color with cterm-only highlights under 'termguicolors':
  when a popup highlight has ctermbg but no guibg, bg_rgb is set to
  CTERMCOLOR (not INVALCOLOR). hl_blend_attr() used this value as a real
  RGB color, producing gray instead of the intended color. Use
  COLOR_INVALID() to detect both INVALCOLOR and CTERMCOLOR, and fall back
  to converting the cterm color number to RGB.

closes: #19943

Signed-off-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>