Further options for grid (object type 17)
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Header - bounding box reserved, set to all zero.
Data
Size
8 double  - grid y-spacing (floating point)
4 int32   - grid y-subdivision
4 int32   - grid y-units (0 = inches, 1 = centimetres)
4 uint32  - grid colour  (Wimp colour 0 - 15)
