### 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(!p_tile_set.is_valid());
	tile_set = p_tile_set;
	atlas_merging_atlases_list->clear();
	for (int i = 0; i < p_tile_set->get_source_count(); i++) {
		int source_id = p_tile_set->get_source_id(i);
		Ref<TileSetAtlasSource> atlas_source = p_tile_set->get_source(source_id);
		if (atlas_source.is_valid()) {
			Ref<Texture2D> texture = atlas_source->get_texture();
			if (texture.is_valid()) {
				String item_text = vformat(TTR("%s (ID: %d)"), texture->get_path().get_file(), source_id);
				atlas_merging_atlases_list->add_item(item_text, texture);
				atlas_merging_atlases_list->set_item_metadata(-1, source_id);
			}
		}
	}
	get_ok_button()->set_disabled(true);
	merge_button->set_disabled(true);
	commited_actions_count = 0;
}
AtlasMergingDialog::AtlasMergingDialog() {
	
	set_title(TTR("Atlas Merging"));