The design coordinates for MM fonts were not rounded. For example,
`FT_Get_Var_Design_Coordinates` returned values with fractional part.
* src/type1/t1load.c (mm_axis_unmap): Refactor with rounding.
* include/freetype/ftmm.h (FT_Var_Axis, FT_Set_Var_Design_Coordinates,
FT_Get_Var_Design_Coordinates): Reword documentation.