This converts a gpar object to a 1-row data.frame representation. This is used when accumulating a data.frame representation to return to the user.
gpar_to_df(gp)gpar object with a single specification. no vectors allowed
data.frame with single row.
Linear and Radial gradient objects are replaced with their first colour.
Although this could be something fancier e.g. list object, or some text
representation of the gradient parameters. Maybe just deparse1()?