The `hdmx` matching can be done before the glyph is loaded.

* include/freetype/internal/tttypes.h (TT_LoaderRec): Add a field.
* src/truetype/ttgload.c (compute_glyph_metrics): Relocate the `hdmx`
code from here...
(tt_loader_init): ... to here, before the glyph is loaded.