Unused ValueFormat fields read as i16 unstead of u16

[wezm]

allsorts/src/layout.rs

Lines 1285 to 1296 in 63c3f48

	if value_format.has_x_placement_device() {
	let _ = ctxt.read_i16be()?;
	}
	if value_format.has_y_placement_device() {
	let _ = ctxt.read_i16be()?;
	}
	if value_format.has_x_advance_device() {
	let _ = ctxt.read_i16be()?;
	}
	if value_format.has_y_advance_device() {
	let _ = ctxt.read_i16be()?;
	}

These fields, whilst unused are offsets (Offset16) which the spec says:

Short offset to a table, same as uint16

So they should be read_u16be. Since they're unused this isn't a problem in practice.