Shapes: Fallback to Geometry backend when function resolving fails

To fill shapes with gradients glProgramPathFragmentInputGenNV is used, which is not present on my system, meaning function resolving fails and nothing is drawn at all (neither shapes with gradients nor shapes without gradients).

Because QQuickNvprFunctions::isSupported() returns true (as the GL_NV_path_rendering extension is present), the renderer does not fallback to the geometry backend.