Don't assert when unsupported types are used in custom materials/effects

Properties inside a custom material or an effect are converted to uniforms, right now we'll assert if we can't convert the type into a uniform. This should be a warning instead and we should add documentation explaining which types are supported and which uniform type they will be converted to (ref. https://doc.qt.io/qt-5/qml-qtquick-shadereffect.html)