Make antialiasing mode configurable

The default Qt Quick items currently use opacity combined with a custom material and additional vertices to do antialiasing. OpenGL supports doing antialiasing through multisampling. On some systems multisampling comes at a high cost and it is not as nice as the vertex-based antialiasing, but on most systems multisample based antialiasing means:

• Antialiased primitives are drawn as opaque, resulting in better batching and reduced blending, resulting in significantly better performance
• Multisampling can be done by the hardware and is "for free"
• Tile-based renderers (such as SGX, Mali) can do multisampling in hardware which means it is for free...

ssh://codereview.qt-project.org:29418/playground/scenegraph.git's customcontext reintroduces the multisample based primitives using the "msaa" config option. We should move this to Qt quick and make it an option on for instance QQuickWindow::setAntialiasingMode() or something like that.