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

libs/imgui/examples/example_sdl2_sdlrenderer2/main.cpp

@@ -59,6 +59,7 @@ int main(int, char**)
     ImGuiIO& io = ImGui::GetIO(); (void)io;
     io.ConfigFlags |= ImGuiConfigFlags_NavEnableKeyboard;     // Enable Keyboard Controls
     io.ConfigFlags |= ImGuiConfigFlags_NavEnableGamepad;      // Enable Gamepad Controls
+    io.ConfigFlags |= ImGuiConfigFlags_DockingEnable;         // Enable Docking
 
     // Setup Dear ImGui style
     ImGui::StyleColorsDark();