Roll ImGui to v1.92.6-docking (#182799)

Preparation for the Clang roll (see
https://github.com/flutter/flutter/issues/182781)

DEPS

diff --git a/DEPS b/DEPS
index eb6fd7a..b74e54a 100644
--- a/DEPS
+++ b/DEPS
@@ -565,7 +565,7 @@
   Var('dart_git') + '/external/github.com/google/vector_math.dart.git' + '@' + '0a5fd95449083d404df9768bc1b321b88a7d2eef', # 2.1.0
 
   'engine/src/flutter/third_party/imgui':
-  Var('flutter_git') + '/third_party/imgui.git' + '@' + '3ea0fad204e994d669f79ed29dcaf61cd5cb571d',
+  Var('flutter_git') + '/third_party/imgui.git' + '@' + '2a1b69f05748ad909f03acf4533447cac1331611',
 
   'engine/src/flutter/third_party/json':
   Var('flutter_git') + '/third_party/json.git' + '@' + '17d9eacd248f58b73f4d1be518ef649fe2295642',

engine/src/flutter/impeller/playground/imgui/imgui_impl_impeller.cc

diff --git a/engine/src/flutter/impeller/playground/imgui/imgui_impl_impeller.cc b/engine/src/flutter/impeller/playground/imgui/imgui_impl_impeller.cc
index 6e7d670..84d0f7a 100644
--- a/engine/src/flutter/impeller/playground/imgui/imgui_impl_impeller.cc
+++ b/engine/src/flutter/impeller/playground/imgui/imgui_impl_impeller.cc
@@ -133,6 +133,11 @@
   auto* bd = ImGui_ImplImpeller_GetBackendData();
   IM_ASSERT(bd != nullptr &&
             "No renderer backend to shutdown, or already shutdown?");
+  ImGuiIO& io = ImGui::GetIO();
+
+  io.BackendRendererName = nullptr;
+  io.BackendRendererUserData = nullptr;
+  io.BackendFlags &= ~ImGuiBackendFlags_RendererHasVtxOffset;
   delete bd;
 }
 

engine/src/flutter/impeller/playground/playground.cc

diff --git a/engine/src/flutter/impeller/playground/playground.cc b/engine/src/flutter/impeller/playground/playground.cc
index 9ca2f35..363c8ba 100644
--- a/engine/src/flutter/impeller/playground/playground.cc
+++ b/engine/src/flutter/impeller/playground/playground.cc
@@ -270,7 +270,7 @@
     RenderTarget render_target = surface->GetRenderTarget();
 
     ImGui::NewFrame();
-    ImGui::DockSpaceOverViewport(ImGui::GetMainViewport(),
+    ImGui::DockSpaceOverViewport(0, ImGui::GetMainViewport(),
                                  ImGuiDockNodeFlags_PassthruCentralNode);
     bool result = render_callback(render_target);
     ImGui::Render();