Some practical debugging work has shown that displaying level X of
an `FT_TRACEX` macro in the output of `FT2_DEBUG="... -v"` would be
very helpful to find out which trace level should be selected. As
an example, we now get output like
```
[ttobjs:2] TTF driver
[ttobjs:2] SFNT driver
[sfobjs:2] not a font using the SFNT container format
[t1objs:2] Type 1 driver
[stream:7] FT_Stream_EnterFrame: 14 bytes
```
* include/freetype/internal/ftdebug.h (FT_LOGGING_TAGX): New macro.
(FT_LOG): Use it to add the trace level to the logging tag.
* include/freetype/internal/fttrace.h (FT_MAX_TRACE_LEVEL_LENGTH):
Adjust.
* docs/DEBUG: Updated.
ff40776591