Rendering: Fix panel PNG rendering when using sub url & serve_from_sub_path = true (#21306)

* Rendering: fix dashboard rendering when using sub url

* rollback on render.go: no need to add sub-path

pkg/api/index.go

@@ -43,7 +43,7 @@ func (hs *HTTPServer) setIndexViewData(c *m.ReqContext) (*dtos.IndexViewData, er
 	appSubURL := setting.AppSubUrl
 
 	// special case when doing localhost call from phantomjs
-	if c.IsRenderCall {
+	if c.IsRenderCall && !hs.Cfg.ServeFromSubPath {
 		appURL = fmt.Sprintf("%s://localhost:%s", setting.Protocol, setting.HttpPort)
 		appSubURL = ""
 		settings["appSubUrl"] = ""