Function parse_layer 

pub fn parse_layer(
    resolver: &Resolver<'_>,
    dict: &PdfDict,
    ocg_id: u32,
    off_set: &HashSet<u32>,
    locked_set: &HashSet<u32>,
    sink: &WarningSink,
) -> Option<Layer>

Parse one OCG dict into a Layer.

Returns None only if /Type is present and is not /OCG; spec- conformant generators always include /Type /OCG, but we tolerate its absence for malformed PDFs that omit it.