Sets the variations field of a font description. OpenType font variations allow to select a font instance by specifying values for a number of axes, such as width or weight.
The format of the variations string is AXIS1=VALUE,AXIS2=VALUE..., with each AXIS a 4 character tag that identifies a font axis, and each VALUE a floating point number. Unknown axes are ignored, and values are clamped to their allowed range.
Pango does not currently have a way to find supported axes of a font. Both harfbuzz or freetype have API for this.