FreeType INSTCTRL behavior

[Po Lu]

As part of the Android port [of GNU Emacs], I've been writing a module
to display TrueType fonts.  This includes a TrueType interpreter for
instructing fonts.

  (Please don't ask why I'm not just using FreeType.  Building
   dependencies for Android is not easy.)

AFAIU, as long as a font does not perform an INSTCTRL which sets bit 2
in the instruction control, changes made to the graphics state from the
control value program will become the ``default'' state from which
glyphs are instructed.

However, some CJK fonts perform no INSTCTRL instructions at all, change
the projection vector and rp1 from prep, yet FreeType resets both to the
X axis and 0 respectively before any glyph programs are run.

Am I missing anything?  If not, could anyone here tell me where this
behavior is documented?

Thanks.