Find a better way to support external images in VectorImage

After the move to use the QML generator internally to also generate scenes at runtime, the VectorImage has started assuming that external image data is on the file system. But a common case might be that you want to load these assets from qrc instead.

It should ideally adapt to find the image relative to the SVG file. One idea is to pass the correct context URL into the QQmlComponent as suggested in a comment to https://codereview.qt-project.org/c/qt/qtdeclarative/+/636232