[truetype] Adjust `ROUND` bytecode instruction for native ClearType.

Microsoft ClearType applies `ROUND[]` to the 1/16-pixel virtual grid in the
ClearType direction.  As with the rounded point-positioning instructions,
approximate this in v40 by leaving the result unrounded during native
ClearType horizontal execution.

This fixes horizontal positioning like those performed by 'Malgun Gothic's
composite glyph programs.  In particular, it corrects diacritic placement in
glyphs such as 'Ÿ' and 'ü' and produces small improvements in several other
accented glyphs.

Other rendering modes and directions retain the existing `ROUND[]` behavior.

* src/truetype/ttinterp.c (Ins_ROUND) [TT_SUPPORT_SUBPIXEL_HINTING_MINIMAL]:
  Handle it.