### 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...
}
				}
			}
		}
	}
}
void AtlasMergingDialog::_update_texture() {
	Vector<int> selected = atlas_merging_atlases_list->get_selected_items();
	if (selected.size() >= 2) {
		Vector<Ref<TileSetAtlasSource>> to_merge;
		for (int i = 0; i < selected.size(); i++) {
			int source_id = atlas_merging_atlases_list->get_item_metadata(selected[i]);
			to_merge.push_back(tile_set->get_source(source_id));
		}
		_generate_merged(to_merge, next_line_after_column);
		preview->set_texture(merged->get_texture());
		preview->show();
		select_2_atlases_label->hide();
		get_ok_button()->set_disabled(false);
		merge_button->set_disabled(false);
	} else {
		_generate_merged(Vector<Ref<TileSetAtlasSource>>(), next_line_after_column);
		preview->set_texture(Ref<Texture2D>());
		preview->hide();
		select_2_atlases_label->show();
		get_ok_button()->set_disabled(true);
		merge_button->set_disabled(true);
	}
}
void AtlasMergingDialog::_merge_confirmed(String p_path) {