prepare method
Prepare the object to be written to the stream
Implementation
@override
void prepare() {
super.prepare();
params['/PatternType'] = PdfNum(patternType);
if (matrix != null) {
final s = matrix!.storage;
params['/Matrix'] =
PdfArray.fromNum(<double>[s[0], s[1], s[4], s[5], s[12], s[13]]);
}
}