VTK adapter for DearImGUI

• An effort to incorporate VTK's dataset visualization capabilities with versatility of DearImGUI.
• Renders VTK content into a dedicated Dear ImGui window. (Like a viewport)
• Use imgui_impl_vtk.cpp, imgui_impl_vtk.h as you would any of `imgui_impl_xxx.h" files.
• Set up pipeline (datasets + filter) with VTK. dataset->filter->...->mapper->actor.
• Init Imgui VTK ImGui_ImplVTK_Init()
• Provide actors to ImGui_ImplVTK_AddActor(vtkProp*)
• Call ImGui_ImplVTK_Render() in event loop.
• Look in main.cpp.

Usage:

dataset = ..
filter = ..
mapper1 = ..
mapper2 = ..
actor1 = ..
actor2 = ..
filter->SetInputData(dataset);

mapper1->SetInputConnection(filter->GetOutputPort(0));
mapper2->SetInputConnection(filter->GetOutputPort(1));
...

actor1->SetMapper(mapper1);
actor2->SetMapper(mapper2);
...

ImguiInit...
ImGui_ImplVTK_Init()

while (!terminate)
{
  Imgui new frame..
  custom imgui windows..
  ImGui_ImplVTK_Render();
  ImGui::Render();
  ImGui_Impl_xxx_RenderDrawData();
}