Represents the "WhitePoint" entry of the calgray colour space
It is a required entry defined as part of the PDF 1.0
It contains an array which contains doubles, representing PDF
Items in this array should have a positive value.
For definitive details see:.
The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 63,
The ISO PDF
Specification, ISO 32000-2:2017 PDF 2.0; Table: 62, page 183.