RecalcAdvances and DoKerning should agree on when to use design metrics

QFontEngineFT::recalcAdvances uses design metrics if hinting is disabled
or slight. QFontEngine::doKerning only follows the QFontEngine::DesignMetrics
flag. This means in some instances the advances will be calculated in
subpixels but kerned in full pixels.

This patch makes QFontEngineFT decide if it should request design metrics
from QFontEngine::doKerning or not.

Change-Id: Ia0236efde2d7269623f690a6074afbe26e07c458
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>
test: Mark tst_qopengl as insignificant on Win7 + Angle

This only marks tst_qopengl as insignificant on Windows 7 32bit
with the Angle configuration.

Task-number: QTBUG-31611
Change-Id: I1876b6fdc32fef93edf34c2bd61d03cc9ba11135
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>