Represents the "IC" entry of the square annotation object.
It is an optional entry defined as part of the PDF 1.4
It contains an array which contains doubles, representing PDF
Items in this array should have a minimum value of 0 and a
maximum of 1.
For definitive details see:.
The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 177,
The ISO PDF
Specification, ISO 32000-2:2017 PDF 2.0; Table: 180, page 487.