Fix the handling of font backend supersedence on MS-Windows

Fix the handling of font backend supersedence on MS-Windows

* src/w32font.c (syms_of_w32font): Don't make the Uniscribe
font backend "superceded" here, ...
* src/w32uniscribe.c (syms_of_w32uniscribe_for_pdumper):
... make it "superceded" here, only if the HarfBuzz DLL was
successfully loaded.  This is because Emacs compiled with
HarfBuzz support might run on a system without the DLL.
* src/w32fns.c (Fx_create_frame, w32_create_tip_frame):
Register the HarfBuzz font backend only if it is available.