Bump Qt version to 4.8.6

Change-Id: I91189c8c33591ef866a4478c113f93162afede95
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>
Fixes: ASSERT in qpaintengine_x11 when using native graphicssystem

QFontEngineFT::loadGlyph() was called with wrong number/type of
arguments, but as the compiler silently accepted an enum value as
a QFixed, and the rest had default values, this was not discovered.

Fix not needed in Qt5, since this functionality is not there.

Task-number: QTBUG-32166
Change-Id: Ifc5be593530fee5ff0e329a9f56f6ba48eea6cdf
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>