[base] Add trace level to logging output.

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):

* docs/DEBUG: Updated.
4 files changed