### Godot version

v4.2.dev3.official [013e8e3af]

### System information

Windows 11, Core i5-12400F, RX 6600

### Issue description

Any images without transparency become RGB8.
Any images with transparency become RGBA8.

When doing a TileSet Atlas Merge RGB8 images do not show up in merge dialog (just empty space) and do not merge.

![image](https://github.com/godotengine/godot/assets/962340/9cd422ce-2f86-41aa-b4f1-97bcba2db003)![Screenshot 2023-08-21 203922](https://github.com/godotengine/godot/assets/962340/905fe87d-c641-4c6e-bd0e-c47fe997c2ff)


### Steps to reproduce

- merge any solid image with no transparency

### Minimal reproduction project

https://github.com/rakkarage/TestTestTest333

EDIT: Added simpler example Test2.tscn, without margin, separation, and using real default size tiles...
ERR_FAIL_COND(!merged.is_valid());
	Ref<ImageTexture> output_image_texture = merged->get_texture();
	output_image_texture->get_image()->save_png(p_path);
	ResourceLoader::import(p_path);
	Ref<Texture2D> new_texture_resource = ResourceLoader::load(p_path, "Texture2D");
	merged->set_texture(new_texture_resource);
	EditorUndoRedoManager *undo_redo = EditorUndoRedoManager::get_singleton();
	undo_redo->create_action(TTR("Merge TileSetAtlasSource"));
	int next_id = tile_set->get_next_source_id();
	undo_redo->add_do_method(*tile_set, "add_source", merged, next_id);
	undo_redo->add_undo_method(*tile_set, "remove_source", next_id);
	if (delete_original_atlases) {
		
		Vector<int> selected = atlas_merging_atlases_list->get_selected_items();
		for (int i = 0; i < selected.size(); i++) {
			int source_id = atlas_merging_atlases_list->get_item_metadata(selected[i]);
			Ref<TileSetAtlasSource> tas = tile_set->get_source(source_id);
			undo_redo->add_do_method(*tile_set, "remove_source", source_id);