Fix rendering of grayscale antialiased FT fonts on QGLWidget

QFontEngineFT::alphaMapForGlyph and QFontEngineFT::alphaRGBMapForGlyph
has been broken since change#65694. They always fall back to using
the path rendering of QFontEngine because we zero the scoped pointer
just before testing it.

To fix it we need to release the scope pointer after we are done using
it.

Change-Id: I8811c1f5261f286f2f3dd3c0f93c988ba0909669
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>