Re: the hdmx stuff

[Werner LEMBERG]

Hello Alexei,

> 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.

OK.

> 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.

I think it shouldn't be a no-op but rather cause an error, no?

```
  if ( exc->iniRange != tt_coderange_cvt )
  {
    exc->error = FT_THROW( ... error ... );
    return;
  }
```


    Werner