ShaderEffectSource: Check width > 0 && height > 0

Following code fails in assertion check in debug mode, but works fine in release:

    Rectangle {
        id: rect
        width: 100
        height: 10
    }

    ShaderEffectSource {
        width: 100
        height: -10
        sourceItem: rect
    }

Please check for '<= 0' of width/height, instead of '== 0' in QQuickShaderEffectSource::updatePaintNode().