Make surface format plugable from the scene graph plugin

Make surface format plugable from the scene graph plugin

Also enable depth, stencil and samples by default as our default
renderer requires both depth and stencil and our default rounded
rectangles require multisampling

The user should be able to override the default format, by getting
the QSGCanvas::format(), modify it and set it back, but this
is currently not supported in QWindow, so that will have to wait

Change-Id: I1869003705709987ab2e9a3cebbeeec4d45a2021
Reviewed-on: http://codereview.qt-project.org/5464
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>