[Qt] Avoid creating a QWindow for offscreen GraphicsContext3D

https://bugs.webkit.org/show_bug.cgi?id=118244

Reviewed by Noam Rosenthal.

In Qt 5.0 we had to create invisible windows, but in Qt 5.1
we can now create specific offscreen surfaces.

* platform/graphics/qt/GraphicsContext3DQt.cpp:
(WebCore::GraphicsContext3DPrivate::GraphicsContext3DPrivate):

Change-Id: Ib5b6adadc1e60ec43883ac61595afbd4e43c5d4a
git-svn-id: http://svn.webkit.org/repository/webkit/trunk@152222 268f45cc-cd09-0410-ab3c-d52691b4dbfc
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>