Problem: Combined highlighting was not applied to nvim_eval_statusline(),
and 'statuscolumn' sign segment/numhl highlights.
Solution: Add an additional `groups` element to the return value of
`nvim_eval_statusline()->highlights`. This is an array of stacked
highlight groups (highest priority last). Also resolve combined
highlights for the 'statuscolumn' sign segment/numhl highlights.
Expose/synchronize some drawline.c logic that is now mimicked in
three different places.
34d808b73c