Shutdown safely and correctly on Apple

Metal and OSX require `ImGui_Impl*_GetBackendData() != nullptr` during
shutdown anyway so this is not a breaking change. Actually, before this
PR there were potential `nullptr` derefs in the respective
`ImGui_Impl*_DestroyBackendData()`s. This PR both prevents this and also
resets `io.BackendRendererUserData`, `io.BackendRendererName` and
`io.BackendFlags`.

backends/imgui_impl_metal.mm

@@ -135,6 +135,12 @@ bool ImGui_ImplMetal_Init(id<MTLDevice> device)
 
 void ImGui_ImplMetal_Shutdown()
 {
+    ImGui_ImplMetal_Data* bd = ImGui_ImplMetal_GetBackendData();
+    IM_ASSERT(bd != nullptr && "No platform backend to shutdown, or already shutdown?");
+    ImGuiIO& io = ImGui::GetIO();
+    io.BackendRendererUserData = nullptr;
+    io.BackendRendererName = nullptr;
+    io.BackendFlags &= ~ImGuiBackendFlags_RendererHasVtxOffset;
     ImGui_ImplMetal_DestroyDeviceObjects();
     ImGui_ImplMetal_DestroyBackendData();
 }

backends/imgui_impl_osx.mm

@@ -476,12 +476,19 @@ bool ImGui_ImplOSX_Init(NSView* view)
 void ImGui_ImplOSX_Shutdown()
 {
     ImGui_ImplOSX_Data* bd = ImGui_ImplOSX_GetBackendData();
+    IM_ASSERT(bd != nullptr && "No platform backend to shutdown, or already shutdown?");
+    ImGuiIO& io = ImGui::GetIO();
+
     bd->Observer = nullptr;
     if (bd->Monitor != nullptr)
     {
         [NSEvent removeMonitor:bd->Monitor];
         bd->Monitor = nullptr;
     }
+
+    io.BackendPlatformName = nullptr;
+    io.BackendPlatformUserData = nullptr;
+    io.BackendFlags &= ~ImGuiBackendFlags_HasMouseCursors;
     ImGui_ImplOSX_DestroyBackendData();
 }