[sdf] Fix compiler warnings.

* src/sdf/ftsdf.c (get_control_box): Use goto instead of letting the next case statement
  execute to avoid compiler warning [-Wimplicit-fallthrough=].

* src/sdf/ftsdf.c (sdf_generate_bounding_box): Fix compiler warning due to signed
  unsigned mismatch.

* src/sdf/ftsdfrend.c (ft_sdf_done): The function must have a return type of `void'
  to match the signature of `FT_Module_Destructor'.
2 files changed