highlighter: do not redefine the same groups several times

previously, it could happen that the same highlighting group was defined
several times, because it was available in several modes within
g:airline#theme[mode]. So the second one would always win.

Therefore, loop through all modes in reverse order and define the group
and remember what group has already been defined. If we happen to have
to re-define the same group, skip it. Since we are traversing the list
in reverse order, this should make sure the last definition wins.

This has the benefit of being more performant and hopefully helps with
e.g. #1779 and similar issues.

autoload/airline/highlighter.vim

@@ -22,6 +22,18 @@ function! s:gui2cui(rgb, fallback)
   return airline#msdos#round_msdos_colors(rgb)
 endfunction
 
+function! s:group_not_done(list, name)
+  if index(a:list, a:name) == -1
+    call add(a:list, a:name)
+    return 1
+  else
+    if &vbs
+      echomsg printf("airline: group: %s already done, skipping", a:name)
+    endif
+    return 0
+  endif
+endfu
+
 function! s:get_syn(group, what)
   if !exists("g:airline_gui_mode")
     let g:airline_gui_mode = airline#init#gui_mode()
@@ -223,7 +235,11 @@ function! airline#highlighter#highlight(modes, ...)
   " draw the base mode, followed by any overrides
   let mapped = map(a:modes, 'v:val == a:modes[0] ? v:val : a:modes[0]."_".v:val')
   let suffix = a:modes[0] == 'inactive' ? '_inactive' : ''
-  for mode in mapped
+  let airline_grouplist=[]
+  " mapped might be something like ['normal', 'normal_modified']
+  " if a group is in both modes available, only define the second
+  " that is how this was done previously overwrite the previous definition
+  for mode in reverse(mapped)
     if exists('g:airline#themes#{g:airline_theme}#palette[mode]')
       let dict = g:airline#themes#{g:airline_theme}#palette[mode]
       for kvp in items(dict)
@@ -232,7 +248,9 @@ function! airline#highlighter#highlight(modes, ...)
         if name is# 'airline_c' && !empty(bufnr) && suffix is# '_inactive'
           let name = 'airline_c'.bufnr
         endif
-        call airline#highlighter#exec(name.suffix, mode_colors)
+        if s:group_not_done(airline_grouplist, name.suffix)
+          call airline#highlighter#exec(name.suffix, mode_colors)
+        endif
 
         for accent in keys(s:accents)
           if !has_key(p.accents, accent)
@@ -250,10 +268,16 @@ function! airline#highlighter#highlight(modes, ...)
           else
             call add(colors, get(p.accents[accent], 4, ''))
           endif
-          call airline#highlighter#exec(name.suffix.'_'.accent, colors)
+          if s:group_not_done(airline_grouplist, name.suffix.'_'.accent)
+            call airline#highlighter#exec(name.suffix.'_'.accent, colors)
+          endif
         endfor
       endfor
 
+      if empty(s:separators)
+        " nothing to be done
+        continue
+      endif
       " TODO: optimize this
       for sep in items(s:separators)
         call <sid>exec_separator(dict, sep[1][0], sep[1][1], sep[1][2], suffix)