Add support for multisampling in QQuickWidget

Use a multisampled fbo when the requested format has samples > 0.
Resolving happens after each rendering of the scene.

The blit to the temporary non-multisampled fbo could be avoided,
in theory, by sending the fbo instead of the texture id down the
stack and performing a blit directly to fbo #0. This however
involves a number of potential issues, for example due to the
non-sharability of FBOs between contexts. Hence it is left as a
future exercise.

Task-number: QTBUG-39187
Change-Id: Iae98b969bcbc3bb57e6d73288496f5428913c826
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>