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

libs/imgui/examples/example_allegro5/README.md

@@ -24,9 +24,9 @@ You may install Allegro using vcpkg:
 git clone https://github.com/Microsoft/vcpkg
 cd vcpkg
 bootstrap-vcpkg.bat
-vcpkg install allegro5 --triplet=x86-windows	; for win32
-vcpkg install allegro5 --triplet=x64-windows	; for win64
-vcpkg integrate install						    ; register include / libs in Visual Studio
+vcpkg install allegro5 --triplet=x86-windows    ; for win32
+vcpkg install allegro5 --triplet=x64-windows    ; for win64
+vcpkg integrate install                         ; register include / libs in Visual Studio
 ```
 
 Build:

libs/imgui/examples/example_allegro5/main.cpp

@@ -40,6 +40,7 @@ int main(int, char**)
     ImGui::CreateContext();
     ImGuiIO& io = ImGui::GetIO(); (void)io;
     io.ConfigFlags |= ImGuiConfigFlags_NavEnableKeyboard;  // Enable Keyboard Controls
+    io.ConfigFlags |= ImGuiConfigFlags_DockingEnable;      // Enable Docking
 
     // Setup Dear ImGui style
     ImGui::StyleColorsDark();