With a low GC threshold the node buffer was being collected (despite guards against this)
and causing further drawing operations to be corrupted.
Add a new tracked malloc method and use it for node buffers and font data.
Tweak pp_poly_add_path to accept a point count, avoiding many successive rellocs when
dealing with a known quantity of points (copying from an af_glyph_t.)
Somewhere in the conversion from C++ to C we lost support for 16-bit point counts.
Raise the internal point count to 16-bit, and check for a flag in the font.
If the flag is set, treat the point count as 16-bits, otherwise fallback to 8 for back compatibility.