Fix issue title rendering and refactor legacy function names (#32703)

Fix #32700, regression of recent markup refactoring

And by the way, clarify many legacy problems:

1. Some "RenderXxx" functions do not really "render", they only call "post processors"
2. Merge "RenderEmoji | RenderCodeBlock", they are all for "simple issue title"

modules/markup/render.go

@@ -57,6 +57,9 @@ type RenderOptions struct {
 type RenderContext struct {
 	ctx context.Context
 
+	// the context might be used by the "render" function, but it might also be used by "postProcess" function
+	usedByRender bool
+
 	SidebarTocNode ast.Node
 
 	RenderHelper   RenderHelper
@@ -182,6 +185,7 @@ func pipes() (io.ReadCloser, io.WriteCloser, func()) {
 }
 
 func render(ctx *RenderContext, renderer Renderer, input io.Reader, output io.Writer) error {
+	ctx.usedByRender = true
 	if ctx.RenderHelper != nil {
 		defer ctx.RenderHelper.CleanUp()
 	}