don't highlight in isearch if ISEARCH_ACTIVE is unsupported

Old version of zsh don't expose ISEARCH_ACTIVE. Therefore we are
unable to re-apply zle_highlight on top and it is impossible to
see the underlined area.

Completely disable highlighting in isearch in that case.

To do that, we need to make sure we are actually called when
something changes in isearch.

Trumps zsh-users#257.

zsh-syntax-highlighting.zsh

@@ -62,6 +62,12 @@ _zsh_highlight()
   # Store the previous command return code to restore it whatever happens.
   local ret=$?
 
+  # Do not highlight in isearch if ISEARCH_ACTIVE is unsupported (zsh < 5.3).
+  if [[ $WIDGET == zle-isearch-update ]] && ! (( $+ISEARCH_ACTIVE )); then
+    region_highlight=()
+    return $ret
+  fi
+
   setopt localoptions warncreateglobal
   setopt localoptions noksharrays
   local REPLY # don't leak $REPLY into global scope
@@ -315,6 +321,10 @@ _zsh_highlight_bind_widgets || {
 # E.g. remove cursor imprint, don't highlight partial paths, ...
 _zsh_highlight_set_or_wrap_special_zle_widget zle-line-finish
 
+# Always wrap special zle-isearch-update widget to be notified of updates in isearch
+_zsh_highlight_set_or_wrap_special_zle_widget zle-isearch-update
+
+
 # Resolve highlighters directory location.
 _zsh_highlight_load_highlighters "${ZSH_HIGHLIGHT_HIGHLIGHTERS_DIR:-${${0:A}:h}/highlighters}" || {
   echo 'zsh-syntax-highlighting: failed loading highlighters, exiting.' >&2