w.r.t. the selected charmap.
* include/freetype/freetype.h: Add prototype.
* include/freetype/internal/ftdriver.h: Add `FTDriver_getNextChar'
typedef.
(FT_Driver_Class): Use it.
* include/freetype/internal/psnames.h: Add `PS_Next_Unicode_Func'
typedef.
(PSNames_Interface): Use it.
* include/freetype/internal/tttypes.h: Add `TT_CharNext_Func'
typedef.
(TT_CMapTable): Use it.
* src/base/ftobjs.c (FT_Get_Next_Char): New function, implementing
high-level API.
* src/cff/cffdrivr.c (cff_get_next_char): New function.
(cff_driver_class): Add it.
* src/cid/cidriver.c (Cid_Get_Next_Char): New function.
(t1cid_driver_class): Add it.
* src/pcf/pcfdriver.c (PCF_Get_Next_Char): New function.
(pcf_driver_class): Add it.
* src/psnames/psmodule.c (PS_Next_Unicode): New function.
(psnames_interface): Add it.
* src/sfnt/ttcmap.c (code_to_next0, code_to_next2, code_to_next4,
code_to_next6, code_to_next_8_12, code_to_next_10): New auxiliary
functions.
(TT_CharMap_Load): Use them.
* src/truetype/ttdriver.c (Get_Next_Char): New function.
(tt_driver_class): Add it.
* src/type1/t1driver.c (Get_Next_Char): New function.
(t1_driver_class): Add it.
* src/winfnt/winfnt.c (FNT_Get_Next_Char): New function.
(winfnt_driver_class): Add it.
* src/pcf/pcfread.c (pcf_load_font): For now, report Unicode for
Unicode and Latin 1 encodings.
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