Glif parsing: ensure outline element has no attributes

[madig]

Minor issue, but we don't do it currently.

norad/src/glyph/parse.rs

Lines 127 to 136 in 2789dbe

	fn parse_outline(
	&mut self,
	reader: &mut Reader<&[u8]>,
	buf: &mut Vec<u8>,
	) -> Result<(), GlifLoadError> {
	let mut outline_builder = OutlineBuilder::new();
	// TODO: Not checking for (the absence of) attributes here because we'd need to
	// pass through the element data, but that'd clash with the mutable borrow of
	// buf. Better way?