Use Qt's own glyph cache with the freetype engine.

This fixes rendering in the OpenGL paint engine,
simplifies our internal code paths and also produces
nicer text for non-90 rotated glyphs

Change-Id: Ib206065cd486ac29067f9ebf4a0d74189a673f1e
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>