Change how Blaze.render,toHTML infer parentView

If you call UI.renderWithData, say, from an event handler, you may be unpleasantly surprised if it gets Blaze.currentView as its parentView (where Blaze.currentView is based on the template where the event handler is defined).  On the other hand, showdown/template-integration.js takes advantage of the fact that Blaze.toHTML in render() is infers the parentView from Blaze.currentView.  So only infer currentView as parent while in the view’s render().

This change should only affect apps and packages that use Blaze.render, Blaze.toHTML, UI.render, or UI.renderWithData.