hugolib: Fix changing paginators in lazy render

Fixes gohugoio#5406

hugolib/hugo_sites.go

@@ -416,6 +416,7 @@ type BuildCfg struct {
 // a Page: If it is recently visited (the home pages will always be in this set) or changed.
 // Note that a page does not have to have a content page / file.
 // For regular builds, this will allways return true.
+// TODO(bep) rename/work this.
 func (cfg *BuildCfg) shouldRender(p *Page) bool {
 	if p.forceRender {
 		p.forceRender = false
@@ -427,6 +428,9 @@ func (cfg *BuildCfg) shouldRender(p *Page) bool {
 	}
 
 	if cfg.RecentlyVisited[p.RelPermalink()] {
+		if cfg.PartialReRender {
+			_ = p.initMainOutputFormat()
+		}
 		return true
 	}
 
@@ -644,15 +648,13 @@ func (h *HugoSites) setupTranslations() {
 
 func (s *Site) preparePagesForRender(start bool) error {
 	for _, p := range s.Pages {
-		p.setContentInit(start)
-		if err := p.initMainOutputFormat(); err != nil {
+		if err := p.prepareForRender(start); err != nil {
 			return err
 		}
 	}
 
 	for _, p := range s.headlessPages {
-		p.setContentInit(start)
-		if err := p.initMainOutputFormat(); err != nil {
+		if err := p.prepareForRender(start); err != nil {
 			return err
 		}
 	}

hugolib/page.go

@@ -304,7 +304,7 @@ func (p *Page) initContent() {
 		go func() {
 			var err error
 
-			err = p.prepareForRender()
+			err = p.prepareContent()
 			if err != nil {
 				c <- err
 				return
@@ -1142,11 +1142,17 @@ func (p *Page) subResourceTargetPathFactory(base string) string {
 	return path.Join(p.relTargetPathBase, base)
 }
 
-func (p *Page) initMainOutputFormat() error {
-	if p.mainPageOutput != nil {
-		return nil
+// Prepare this page for rendering for a new site. The flag start is set
+// for the first site.
+func (p *Page) prepareForRender(start bool) error {
+	p.setContentInit(start)
+	if start {
+		return p.initMainOutputFormat()
 	}
+	return nil
+}
 
+func (p *Page) initMainOutputFormat() error {
 	outFormat := p.outputFormats[0]
 	pageOutput, err := newPageOutput(p, false, false, outFormat)
 
@@ -1193,7 +1199,7 @@ func (p *Page) setContentInit(start bool) error {
 
 }
 
-func (p *Page) prepareForRender() error {
+func (p *Page) prepareContent() error {
 	s := p.s
 
 	// If we got this far it means that this is either a new Page pointer