The `hdmx` table is supposed to be sorted by ppem size, which
enables binary search.  We also drop the check for the sufficient
length of the record because it is now enforced when the table
is loaded.

* include/freetype/internal/tttypes.h (TT_FaceRec): Store the `hdmx`
record pointers sorted by ppem instead of ppem's themselves.
* src/truetype/ttpload.c (tt_face_load_hdmx): Prudently sort records.
(tt_face_get_device_metrics): Implement binary search to retrieve
advances.