Add integration with OpenType VF `ital` and `slnt` axes
Map the FC_SLANT
setting to either the ital
or slnt
axis depending
on what is available in the font. This allows client applications with
FC_FONT_VARIATIONS
support to specify italic or oblique font
variations through fontconfig.
This also introduce a new setting called FC_SLANT_ANGLE
which is
mapped directly to the value of the slnt
axis when available. This
allows the user to select which slant angle to be used for oblique
variations.