ImGui: Support OSG threading models other than SingleThreaded

[gwaldron]

Some of the ImGui components do not work unless OSG is in SingleThreaded mode. SingleThreaded mode impacts performance; you can measure by running osgearth_viewer --novsync --SingleThreaded and checking the relative frame rates.

Task: Identify and update the non-working ImGui components so they work in DrawThreadPerContext et al.

[jasonbeverage]

I think we can separate the pre draw imgui stuff and put it in an update traversal but leave the actual render call where it is.