This achieves the desired behavior, but extending ImGuiMenu creates a bit of a mess. I previously had the impression that Viewer::plugins list could contain multiple ImGui menus/plugins. That appears to be impossible because of how the plugin calls pre_draw/post_draw: you can only have one ImGuiMenu/ImGui-based plugin. I'm wondering if anyone is actually using more than one plugin...

I noticed this when making an app that tried to use two of these ImGuizmoPlugins. ImGui crashes when on the pre_draw of the second plugin:

-> 6733 IM_ASSERT((g.FrameCount == 0 || g.FrameCountEnded == g.FrameCount) && "Forgot to call Render() or EndFrame() at the end of the previous frame?");

Having the caller fill out a class (like the old demo) shifts this problem to the user who could solve it by putting 2+ guizmos in the post_draw. But this is really cumbersome.

I like what this PR achieves. The caller simply pushes the plugin to Viewer::plugins and attaches a callback.

ImGuizmo doesn't appear support multiple guizmos so the issue above is moot (still think the plugin predraw/postdraw could be an issue, but I'll just leave it crossed out here for reference in case I forget this and find it later).

As far as ImGuizmos, if the caller wants to use the guizmo for different things then they need to handle the logic for that.