sdf: revise memory allocation debugging

[Werner LEMBERG]

Anuj,


I think that the memory debugging functions in file `ftsdf.c` can be
greatly simplified.  Since `FT_DEBUG_MEMORY` must be defined I can't
see a reason to not use the default FreeType debugging stuff.  In
other words, `SDF_ALLOC` and friends can be replaced with `FT_ALLOC`
and friends (more or less).

What do you think?  If you agree, could you prepare a patch?


    Werner