hugolib: Handle shortcode per output format

This commit allows shortcode per output format, a typical use case would be the special AMP media tags.

Note that this will only re-render the "overridden" shortcodes and only  in pages where these are used, so performance in the normal case should not suffer.

Closes #3220

hugolib/hugo_sites_build.go

@@ -213,7 +213,7 @@ func (h *HugoSites) render(config *BuildCfg) error {
 		s.initRenderFormats()
 		for i, rf := range s.renderFormats {
 			s.rc = &siteRenderingContext{Format: rf}
-			s.preparePagesForRender(i, config)
+			s.preparePagesForRender(config)
 
 			if !config.SkipRender {
 				if err := s.render(i); err != nil {