expose draw list user callback

[shakesoda]

I've got a bunch of custom views, but without a way to set callbacks on the draw lists I have to draw them to a canvas and draw them as images... it's awkward and a lot slower than just being able to give it a draw function.

^ this involves a ton of canvases, but most of them wouldn't be needed if I had a render callback (I'd just need the one for shaded view)

short of fully exposing ImDrawList, something like imgui.AddCallback(fn) which just gets the window draw list and adds the callback (i.e. GetWindowDrawList()->AddCallback(...)) would be wonderful