Remove deprecated Fonts->Build() and clean up ImGui texture lifecycle

* Remove the explicit io.Fonts->Build() call from InitImGui, which was deprecated in Dear ImGui v1.92.0. With ImGuiBackendFlags_RendererHasTextures already set, the font atlas is now built lazily by ImGui and communicated to backends via the ImTextureStatus_WantCreate protocol.

* Add proper texture shutdown in ~ImguiBridge by iterating ImGui::GetPlatformIO().Textures and destroying all tracked textures, per the //third_party/dear_imgui/docs/BACKENDS.md instructions.

* Remove the "OK but missing" and "WantUpdates but missing" texture recovery hacks from Update(). These handled a state where ImGui thought textures were alive but the Filament-side storage had been torn down. With proper destruction acknowledgement (SetStatus(Destroyed) + SetTexID(Invalid)), ImGui will now transition destroyed textures to WantCreate on the next frame, and the normal creation path handles it cleanly.

PiperOrigin-RevId: 905886622
Change-Id: Ic449eeaf083740a67eae9c637fb53546a6192b28

src/experimental/platform/hal/window.cc

@@ -47,8 +47,8 @@ extern void* GetNativeWindowOsx(void* window);
 
 namespace mujoco::platform {
 
-static void InitImGui(SDL_Window* window, float content_scale, bool load_fonts,
-                      bool build_fonts) {
+static void InitImGui(SDL_Window* window, float content_scale,
+                      bool load_fonts) {
   ImGui::CreateContext();
 
   ImGuiIO& io = ImGui::GetIO();
@@ -85,10 +85,6 @@ static void InitImGui(SDL_Window* window, float content_scale, bool load_fonts,
     constexpr ImWchar icon_ranges[] = {0xf000, 0xf3ff, 0x000};
     io.Fonts->AddFontFromMemoryTTF(data, size, 14.f, &icon_cfg, icon_ranges);
 
-    if (build_fonts) {
-      io.Fonts->Build();
-    }
-
     // Note: we purposefully do not "close" the font resources as ImGui may
     // need them again to resize fonts.
   }
@@ -133,9 +129,7 @@ Window::Window(std::string_view title, int width, int height, Config config)
     mju_error("Error creating window: %s", SDL_GetError());
   }
 
-  InitImGui(sdl_window_, content_scale, config.load_fonts,
-            (config_.gfx_mode != GraphicsMode::ClassicOpenGl &&
-             config_.gfx_mode != GraphicsMode::ClassicOpenGlHeadless));
+  InitImGui(sdl_window_, content_scale, config.load_fonts);
 
   // Filament (except WebGL) manages its own swap chain including when to swap.
   // In all other cases, we'll use SDL to manage the swap chain.