s = pfontmap:GetSerial()
The serial number is initialized to an small number larger than zero when a new fontmap is created and is increased whenever the fontmap is changed. It may wrap, but will never have the value 0. Since it can wrap, never compare it with "less than", always use "not equals".
The fontmap can only be changed using backend-specific API, like changing fontmap resolution.
This can be used to automatically detect changes to a Pango font map, like in Pango context.
This function returns the current serial number of the fontmap.