Move common HarfBuzz code to a common file hbfont.c

Move common HarfBuzz code to a common file hbfont.c

* src/hbfont.c: New file, with code moved from w32uniscribe.c
and renamed/modified as appropriate.
* src/w32uniscribe.c: Move to hbfont.c DEF_DLL_FN and macro
definitions for HarfBuzz functions used in hbfont.c
(load_harfbuzz_funcs): Move loading of HarfBuzz functions used
by hbfont.c to hbfont.c:hbfont_init_w32_funcs, and call that
function from here.
(syms_of_w32uniscribe_for_pdumper): Fill the 'shape' and
'combining_capability' members with hbfont.c function names.
* src/w32common.h (hbfont_init_w32_funcs) [HAVE_HARFBUZZ]: Add
prototype.
* src/font.h (hbfont_shape, hbfont_combining_capability)
[HAVE_HARFBUZZ]: Add prototypes.
* src/Makefile.in (SOME_MACHINE_OBJECTS): Add hbfont.o.

* configure.ac (FONT_OBJ): Add hbfont.o if HAVE_HARFBUZZ.