Fallback to native font rendering for bitmap fonts.

Distance field rendering requires glyph outlines, which are not
available for bitmap fonts.

Task-number: QTBUG-32737
Change-Id: I64569e4e6b2285ca998b5f80ec31ab04cf1f3d09
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>