A `PangoCoverage` structure is a map from Unicode characters to [enum@Pango.
CoverageLevel] values.
It is often necessary in Pango to determine if a particular font can represent a particular character, and also how well it can represent
that character. The `PangoCoverage` is a data structure that is used to represent that information. It is an opaque structure with no
public fields.
Set the coverage for each index in this
to be the max (better) value of the current coverage for the index and the coverage for the corresponding index in other
.