[HarfBuzz] The canonical ordering of hamza marks

[Khaled Hosny]

Hi Behdad,

I just noticed that hamza above and below marks (U+0654, U+0655), are
reordered after Arabic vowel marks because they have much higher
combining classes, 230 and 222 (the "generic" combining class above and
below, respectively), which breaks mkmk positioning.

I tried to make a patch to handle them like the shadda, but I'm not sure
how safe it is with such generic combining classes.

Regards,
 Khaled