Replace XChar2b with unsigned in all font backends

[martin rudalics]

After this commit, building master on Windows here produces



  CC       w32term.o
../../src/w32font.c: In function 'w32font_draw':
../../src/w32font.c:708:25: warning: passing argument 6 of 'ExtTextOutW' from 
incompatible pointer type [-Wincompatible-pointer-types]
        s->char2b + from + i, 1, NULL);
        ~~~~~~~~~~~~~~~~~^~~
In file included from C:/msys64/mingw64/x86_64-w64-mingw32/include/windows.h:71,
                 from ../../src/w32font.c:20:
C:/msys64/mingw64/x86_64-w64-mingw32/include/wingdi.h:3347:100: note: expected 
'LPCWSTR' {aka 'const short unsigned int *'} but argument is of type 'unsigned 
int *'
   WINGDIAPI WINBOOL WINAPI ExtTextOutW(HDC hdc,int x,int y,UINT options,CONST 
RECT *lprect,LPCWSTR lpString,UINT c,CONST INT *lpDx);
                                                                                
            ~~~~~~~~^~~~~~~~
../../src/w32font.c:711:57: warning: passing argument 6 of 'ExtTextOutW' from 
incompatible pointer type [-Wincompatible-pointer-types]
     ExtTextOutW (s->hdc, x, y, options, NULL, s->char2b + from, len, NULL);
                                               ~~~~~~~~~~^~~~~~
In file included from C:/msys64/mingw64/x86_64-w64-mingw32/include/windows.h:71,
                 from ../../src/w32font.c:20:
C:/msys64/mingw64/x86_64-w64-mingw32/include/wingdi.h:3347:100: note: expected 
'LPCWSTR' {aka 'const short unsigned int *'} but argument is of type 'unsigned 
int *'
   WINGDIAPI WINBOOL WINAPI ExtTextOutW(HDC hdc,int x,int y,UINT options,CONST 
RECT *lprect,LPCWSTR lpString,UINT c,CONST INT *lpDx);
                                                                                
            ~~~~~~~~^~~~~~~~
  CC       w32xfns.o



and the resulting build is not usable.

martin