Function render_shading 

pub fn render_shading(
    pixmap: &mut Pixmap,
    shading_dict: &PdfDict,
    ctm: &Matrix,
    page_transform: &Matrix,
    clip_mask: Option<&Mask>,
    stream_data: Option<&[u8]>,
)

Render a shading pattern into the device. stream_data is the decoded binary stream for mesh shadings (Type 4/5/6/7).