### 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...
undo_redo->add_undo_method(*tile_set, "add_source", tas, source_id);
			
			for (int tile_index = 0; tile_index < tas->get_tiles_count(); tile_index++) {
				Vector2i tile_id = tas->get_tile_id(tile_index);
				undo_redo->add_do_method(*tile_set, "set_coords_level_tile_proxy", source_id, tile_id, next_id, merged_mapping[i][tile_id]);
				if (tile_set->has_coords_level_tile_proxy(source_id, tile_id)) {
					Array a = tile_set->get_coords_level_tile_proxy(source_id, tile_id);
					undo_redo->add_undo_method(*tile_set, "set_coords_level_tile_proxy", a[0], a[1]);
				} else {
					undo_redo->add_undo_method(*tile_set, "remove_coords_level_tile_proxy", source_id, tile_id);
				}
			}
		}
	}
	undo_redo->commit_action();
	commited_actions_count++;
	hide();
}
void AtlasMergingDialog::ok_pressed() {
	delete_original_atlases = false;
	editor_file_dialog->popup_file_dialog();
}
void AtlasMergingDialog::cancel_pressed() {
	EditorUndoRedoManager *undo_redo = EditorUndoRedoManager::get_singleton();
	for (int i = 0; i < commited_actions_count; i++) {
		undo_redo->undo();
	}
	commited_actions_count = 0;
}
void AtlasMergingDialog::custom_action(const String &p_action) {