This makes the determination of whether or not to use fast shaping
platform independent. Previously it was less stringent for Windows,
leading to using it in cases where a font had a GSUB or GPOS table --
which broke proper shaping.
In addition, the test is made platform independent and expanded to be
more complete.
Finally, comments are added indicating that "fast shaping" will be
removed.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
