Problem:
1. Setting `pumborder=+,+,+,+,+,+,+,+` failed to render the custom
border characters correctly. The issue occurred in `parse_winborder()`
where it incorrectly used `p_winborder` instead of the `border_opt`
parameter when the option value didn't contain commas.
2. In `pum_redraw()`, calling `parse_border_style()` directly with the
option string failed to parse comma-separated border characters.
3. Missing documentation for PmenuShadow and PmenuShadowThrough
highlight groups used by the shadow border style.
4. Coverity reports CID 631420: passing WinConfig (480 bytes) by value
in `grid_draw_border()`.
5. crash when using `shadow` value on pumborder.
Solution:
1. Fix `parse_winborder()` to use `border_opt` parameter consistently,
ensuring the correct option value is parsed regardless of which
option (winborder/pumborder) is being set.
2. Update `pum_redraw()` to call `parse_winborder()` instead of
`parse_border_style()`, properly handling both predefined styles
and custom comma-separated border characters.
3. Add documentation for PmenuShadow (blended shadow areas) and
PmenuShadowThrough (see-through corners) highlight groups.
4. Change `grid_draw_border()` to accept WinConfig by pointer.
5. When the "shadow" style is used, no additional row and column offset
is applied, and the border width is reduced.
072f126453