Fields of read-only valuetypes can be written

This correctly gives an error (AnimatedImage sourceSize is read-only):

AnimatedImage {
    sourceSize: "100x100"
}

whereas this does not:

AnimatedImage {
    sourceSize.width: 100
    sourceSize.height: 100
}

Reproduced in tst_qdeclarativevaluetypes::sizereadonly().