Allow GSFont object to be passed to build_masters directly (in place of a path)

[Hoolean]

This helps avoid a costly and redundant save-and-load for Glyphs sources that are manipulated in memory prior to conversion.

[anthrotype]

i'm not necessarily opposed to this, however note that this build_masters function will continue to interact with the filesystem (it writes out the DS and the master UFOs to disk). If you don't want to do that, wouldn't it be better for your code to actually call the inner to_designspace function which this build_masters uses?

[anthrotype]

i suppose this is still useful if one wants do to do what build_masters does but also wants to do some extra pre-processing on the GSFont before it is turned into DS+UFO, so LGTM

[Hoolean]

yes, that last comment is our exact use-case 🚀

[Hoolean]

Ta for speedy review Cosimo :) I don't have merge rights here, so I'm afraid I'll need to ask a maintainer to hit the final merge button for me too