Gets the transfer function applied to the device colour after the conversion, or null for the device's
default (none). An identity transfer function changes nothing.
public PdfTransferFunction? TransferFunction { get; init; }
ISO 32000-2 §10.5 and Table 57 (TR, TR2; deprecated in PDF 2.0). Each device component goes through its own
function in additive form: an RGB device uses the first three of four functions, a gray device the fourth, a CMYK device all
four (a subtractive component c passes as 1 − c and the result is converted back); a single function applies to every
component. A DeviceGray colour converted to CMYK passes only its black through the gray function.