Switch to imgui docking branch, vendor it

src/renderer/renderer_dx12.cpp

@@ -276,6 +276,7 @@ static void init_imgui(Renderer *r)
     ImGui::CreateContext();
     ImGuiIO &io = ImGui::GetIO();
     io.ConfigFlags |= ImGuiConfigFlags_NavEnableKeyboard;
+    io.ConfigFlags |= ImGuiConfigFlags_DockingEnable;
 
     ImGui::StyleColorsDark();