More axes for `ftmulti`

[Werner LEMBERG]

I've just increased the number of handled axes in `ftmulti` from 6 to
15.  Due to the new support of the 'avar' version 2 OpenType extension
it is expected that the number of axes in many fonts will increase
(after this extension gets accepted in the OpenType standard) because
adding 'virtual axes', which simply warp existing axis data, is cheap
now.

Behdad, please have a look – a quick test seems to deliver the same
results as `hb-view`.


    Werner