QFontEngine: Read minimum left and right glyph bearings from 'hhea' table

This table has values precomputed based on every single glyph in
the font, not just the subset we use as a fallback, which should
improve both performance and correctness.

The fallback codepath of computing the minimum values based on a
subset of the characters in the font is left in, as we still need
that for bitmap fonts, and some font tables that have invalid
values.

Change-Id: I71aac1e09c9f9de80446b023ba15a9e2afe7d226
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>