Quick item layers with user-specified MSAA, as an alternative to supersampling

Item layers allow going through an FBO for a subtree. Would be nice if there would be a way to request a given sample count which would then lead to using a multisample renderbuffer, when supported. This way some subtrees could be rendered with MSAA without enabling MSAA for the entire window.

The support is pretty much there in QSGDefaultLayer. We just need to make the sample count configurable via a public QML API.