MultiSelect: ImGuiSelectionBasicStorage: (breaking) rework GetNextSelectedItem() api to avoid ambiguity/failure when user uses a zero id.

imgui.h

@@ -2817,7 +2817,7 @@ struct ImGuiSelectionRequest
 
 // Optional helper to store multi-selection state + apply multi-selection requests.
 // - Used by our demos and provided as a convenience to easily implement basic multi-selection.
-// - Iterate selection with 'void* it = NULL; while (ImGuiId id = selection.GetNextSelectedItem(&it)) { ... }'
+// - Iterate selection with 'void* it = NULL; ImGuiID id; while (selection.GetNextSelectedItem(&it, &id)) { ... }'
 //   Or you can check 'if (Contains(id)) { ... }' for each possible object if their number is not too high to iterate.
 // - USING THIS IS NOT MANDATORY. This is only a helper and not a required API.
 // To store a multi-selection, in your application you could:
@@ -2834,10 +2834,10 @@ struct ImGuiSelectionRequest
 struct ImGuiSelectionBasicStorage
 {
     // Members
-    ImGuiStorage    _Storage;       // [Internal] Selection set. Think of this as similar to e.g. std::set<ImGuiID>. Prefer not accessing directly: iterate with GetNextSelectedItem().
     int             Size;           // Number of selected items (== number of 1 in the Storage), maintained by this helper.
     void*           UserData;       // User data for use by adapter function                // e.g. selection.UserData = (void*)my_items;
     ImGuiID         (*AdapterIndexToStorageId)(ImGuiSelectionBasicStorage* self, int idx);  // e.g. selection.AdapterIndexToStorageId = [](ImGuiSelectionBasicStorage* self, int idx) { return ((MyItems**)self->UserData)[idx]->ID; };
+    ImGuiStorage    _Storage;       // [Internal] Selection set. Think of this as similar to e.g. std::set<ImGuiID>. Prefer not accessing directly: iterate with GetNextSelectedItem().
 
     // Methods
     ImGuiSelectionBasicStorage();
@@ -2846,7 +2846,7 @@ struct ImGuiSelectionBasicStorage
     IMGUI_API void  Clear();                                    // Clear selection
     IMGUI_API void  Swap(ImGuiSelectionBasicStorage& r);        // Swap two selections
     IMGUI_API void  SetItemSelected(ImGuiID id, bool selected); // Add/remove an item from selection (generally done by ApplyRequests() function)
-    IMGUI_API ImGuiID GetNextSelectedItem(void** opaque_it);    // Iterate selection with 'void* it = NULL; while (ImGuiId id = selection.GetNextSelectedItem(&it)) { ... }'
+    IMGUI_API bool  GetNextSelectedItem(void** opaque_it, ImGuiID* out_id); // Iterate selection with 'void* it = NULL; ImGuiId id; while (selection.GetNextSelectedItem(&it, &id)) { ... }'
     inline ImGuiID  GetStorageIdFromIndex(int idx)              { return AdapterIndexToStorageId(this, idx); }  // Convert index to item id based on provided adapter.
 };