the hdmx stuff

[Alexei Podtelezhnikov]

Hi Werner,

To make `hdmx` easily accessible, I propose the following.

1) Use internal FT_LOAD_ADVANCE_ONLY in truetype to quickly exit if
`hdmx` is deemed usable and size matches.
2) "quickly" means from tt_loader_init. We will relocate the bulk of
the `hdmx` code there from compute_glyph_metrics.
3) INSTCTRL needs to be confined to CVT as OpenType specs require.

Do you know how to make INSTCTRL no-op anywhere except CVT? Otherwise
it is rather strange to reconsider each time TT_RunIns runs under v40.

Thanks
Alexei