gravityToRotation
Converts a PangoGravity value to its natural rotation in radians.
Note that method@Pango.Matrix.rotate takes angle in degrees, not radians. So, to call method@Pango.Matrix,rotate with the output of this function you should multiply it by (180. / G_PI).
Return
the rotation value corresponding to @gravity.
Since
1.16
Parameters
gravity
gravity to query, should not be %PANGO_GRAVITY_AUTO