qt: prevent crash when OpenVG scene graph backend is used

When OpenVG is used, the rectangle node provided does not offer a material. At the same time, it is also not a null pointer but it explicitly points to memory address "1" as a workaround done by Qt [1].

Obviously, that memory address does not contain a real QSGMaterial instance, so dereferencing the material causes segmentation fault.

[1] https://github.com/qt/qtdeclarative/blob/6.7.2/src/plugins/scenegraph/openvg/qsgopenvginternalrectanglenode.cpp#L12