qt: add option to request MSAA

Depending on the rendering device, using multi-sample anti aliasing instead of vertex AA can be more beneficial.

Rounded rectangles and some other primitives, including the custom RoundImage become opaque if MSAA is active, and rendering these do not require blending to be enabled. This also increase the chances of batching.

https://doc.qt.io/qt-6/qtquick-visualcanvas-scenegraph-renderer.html#vertex-antialiasing https://doc.qt.io/qt-6/qtquick-visualcanvas-scenegraph-renderer.html#multisample-antialiasing

The option is by default (-1), which means MSAA is not requested. Maybe in future this can be changed to 4, which seems to be good balance between performance and quality.