This flag activates `native ClearType hinting', disabling backwards
compatibility mode as described in Greg Hitchcocks whitepaper. In
other words, it enables unrestricted functionality of all TrueType
instructions in ClearType.
* src/truetype/ttgload.c (tt_get_metrics): Call `sph_set_tweaks'
unconditionally.
(tt_loader_init): Unset `ignore_x_mode' flag if bit 2 of
`GS.instruct_control' is active.
* src/truetype/ttinterp.c (Ins_INSTCTRL): Handle selector index 3.
(Ins_GETINFO): Updated.
* docs/CHANGES: Document it.
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