When true, costly content waits until its element first comes within one viewport height
of the visible area (see RunWhenNearViewport(Control, Action)): images start
loading (unless already cached), formulas and Mermaid diagrams render, and code blocks, shown as
plain text until then, get their colours. When false, formulas and colours are built during
Markdig.Renderers.RendererBase.Render(Markdig.Syntax.MarkdownObject), and images and diagrams start as soon as their element is
attached to a visual tree. Defaults to false, since the output of a renderer may never be
laid out in a viewport; MarkdownViewer turns it on from its
IsLazyRenderingEnabled.
public bool IsLazyRenderingEnabled { get; set; }