Sort pagemarks correctly when adding HTML pagenums

Although `markNext` iterates through the marks in order of their
location, it does not guarantee an order when two marks have the
same location. This could lead to some page numbers that directly
followed blank pages not being output to the HTML file.

To resolve it, just sort a list of page markers and use that, rather
than iterating through the markers in the file.

Fixes #1272

src/lib/Guiguts/HTMLConvert.pm

@@ -1245,20 +1245,19 @@ sub html_convert_sidenotes {
 sub html_convert_pageanchors {
     my $textwindow = $::textwindow;
     ::working("Inserting Page Number Markup");
-    $|++;
     my @pagerefs;    # keep track of first/last page markers at the same position
-    my $tempcounter;
-    my $mark = '1.0';
-    while ( $textwindow->markPrevious($mark) ) {
-        $mark = $textwindow->markPrevious($mark);
-    }
 
     my $closure = voidclosure();
 
-    # Work through all the text markers
-    while ( $mark = $textwindow->markNext($mark) ) {
-        next unless $mark =~ m{Pg(\S+)};              # Only look at page markers
-        my $markindex = $textwindow->index($mark);    # Get page marker's index
+    # Get a correctly sorted list of page markers - if two markers are at the
+    # same location, the order returned by markNext is not defined, but we need
+    # them ordered by png.
+    my @pagemarklist = sort grep ( /^Pg\S+$/, $textwindow->markNames );
+
+    for my $idx ( 0 .. $#pagemarklist ) {
+        my $mark      = $pagemarklist[$idx];
+        my $markindex = $textwindow->index($mark);                                # Get page marker's index
+        my $marknext  = $idx < $#pagemarklist ? $pagemarklist[ $idx + 1 ] : "";
 
         # This is the custom page label
         my $num = $::pagenumbers{$mark}{label};
@@ -1278,12 +1277,6 @@ sub html_convert_pageanchors {
             push @pagerefs, $num;
         }
 
-        # Find next page marker & its index
-        my $marknext = $mark;
-        while ( $marknext = $textwindow->markNext($marknext) ) {
-            last if $marknext =~ m{Pg(\S+)};
-        }
-
         # If no more marks (reached end of file) or there are word characters between the
         # current mark and the next, then convert batch of accumulated page markers to a string
         my $pagereference = '';