It's easier to have stubs if FT_DEBUG_LOGGING is undefined than to
modify `ftexport.sym` conditionally.
Problem reported by Alexei.
* src/base/ftdebug.c: Include `ftlogging.h`.
(FT_Trace_Set_Level, FT_Trace_Set_Default_Level, FT_Set_Log_Handler,
FT_Set_Default_Log_Handler) [!FT_DEBUG_LOGGING]: Provide stubs.
* 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.
* include/freetype/ftlogging.h: New header file.
* include/freetype/internal/ftdebug.h [FT_LOGGING]: Include
`ftlogging.h`.
* src/base/ftdebug.c (ft_custom_trace_level): New variable.
(ft_debug_init): Update to support change of levels of tracing
components of FreeType at run-time.
(FT_Trace_Set_Level): New function to change the levels of tracing
components at run-time.
(FT_Trace_Set_Default_Level): New function to reset the levels of
tracing components back to default.
Werner Lemberg