* include/freetype/ftlogging.h (FT_Custom_Log_Handler): New function
typedef to store the custom callback logging function.
(FT_Set_Log_Handler, FT_Set_Default_Log_Handler): New functions to
set and reset custom log handler.
* include/freetype/internal/ftdebug.h (custom_output_handler): New
variable to support a custom callback logging function.
(FT_Logging_Callback): A new function typedef to print log using
custom callback logging function, which is set using
`FT_Set_Log_Handler`.
(FT_Log): Use it.
* src/base/ftdebug.c (FT_Set_Log_Handler,
FT_Set_Default_Log_Handler, FT_Logging_Callback): Add function
definitions.
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