Constant FRAME_DEMAND_TOKEN_STILL_IMAGE 

pub const FRAME_DEMAND_TOKEN_STILL_IMAGE: u64 = sys::MLN_FRAME_DEMAND_TOKEN_STILL_IMAGE; // 18_446_744_073_709_551_615u64

The token of the frame results that a session adds when it renders an image for MapHandle::request_still_image(). Your own demands can’t use it.

See MLN_FRAME_DEMAND_TOKEN_STILL_IMAGE in the C API reference.