Move min left/right bearing calculations to QFontEngine baseclass

The logic used in the FreeType font engine can be generalized
and move to the QFontEngine baseclass. This allows the CoreText
font engine to correctly report the minimum left/right bearings,
which decreases the chance that an optimization in QTextLayout's
line breaking algorithm will produce wrong results.

The calculation of left and right bearing has been moved to the
glyph_metrics_t type to reduce code duplication. This allows us
to use the with and height of the bounding box to determine if
the glyph has any contours.

Change-Id: I864697d3f31ed56f22f04666199b6c5023c5e585
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>