Get rid of some duplicate code in QFontEngineFT

This removes some code duplication in the QFontEngineFT and
it also removes the condition that loadTransformedGlyphSet()
will return 0 if the font is too large to be cached, since
this is handled by setting outline_drawing on the glyph set which
will fall back to painter paths.

Change-Id: I2fe6a963e854cdd7c6015154547dc06325cbe3b0
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>