Make OpenGL texture glyph cache use the right freetype glyphs

The freetype font engine would fall back to rasterizing glyphs
via QPainterPath in QFontEngine::alphaMapForGlyph() which has
a rather unfortunate implementation.

Change-Id: Ic0b4095b6f17ab33f0602139f0a8fb441dfba0ee
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>