Canvas 2D arcTo assumes current transform is invertible

[junov]

The steps for the arcTo method of CanvasPathMethods require inverting the current transform matrix, which may be non-invertible. The behavior in the case where the matrix is non-invertible is not currently specified. Furthermore, a current transform matrix may not even exist, for example when building a Path2D object. The step involving the CTM needs to have some kind of branch to handle that case.