Recalculate glyph metrics for variable fonts

[David Colson]

I am trying to adjust the weight of a variable font, Source Code Pro in
particular (https://fonts.google.com/specimen/Source+Code+Pro).

I simply call FT_Set_Var_Design_Coordinates with the appropriate
parameters, and it basically works, however, it seems to break all the
glyph metrics and the whole line of text is displayed with overlapping
glyphs, like the advance has become incorrect.

This only happens when setting a design coordinate. Do I need to
recalculate the metrics or something after changing a variable? I had a
look around the docs but could not figure out what I'm doing wrong.
Currently using FreeType 2.10.1.

Thanks