update dear imgui from 1.91.8 to 1.91.9b and switch to docking branch

libs/imgui/backends/imgui_impl_osx.h

@@ -10,6 +10,11 @@
 //  [X] Platform: Gamepad support. Enabled with 'io.ConfigFlags |= ImGuiConfigFlags_NavEnableGamepad'.
 //  [X] Platform: Mouse cursor shape and visibility (ImGuiBackendFlags_HasMouseCursors). Disable with 'io.ConfigFlags |= ImGuiConfigFlags_NoMouseCursorChange'.
 //  [X] Platform: IME support.
+//  [x] Platform: Multi-viewport / platform windows.
+// Missing features or Issues:
+//  [ ] Missing ImGuiMouseCursor_Wait and ImGuiMouseCursor_Progress cursors.
+//  [ ] Multi-viewport: Window size not correctly reported when enabling io.ConfigViewportsNoDecoration
+//  [ ] Multi-viewport: ParentViewportID not honored, and so io.ConfigViewportsNoDefaultParent has no effect (minor).
 
 // You can use unmodified imgui_impl_* files in your project. See examples/ folder for examples of using this.
 // Prefer including the entire imgui/ repository into your project (either as a copy or as a submodule), and only build the backends you need.
@@ -19,6 +24,7 @@
 // - Documentation        https://dearimgui.com/docs (same as your local docs/ folder).
 // - Introduction, links and more at the top of imgui.cpp
 
+#pragma once
 #include "imgui.h"      // IMGUI_IMPL_API
 #ifndef IMGUI_DISABLE