Computes (alpha*Sigma1^-1+(1-alpha)*Sigma2^-1)^-1*
alpha*(Sigma1^-1*mu1)+(1-alpha)*(Sigma2^-1*mu2)) as required for the
computation of the ridgeline (Ray and Lindsay, 2005) to find
all density extrema of a two-component Gaussian mixture with
mean vectors mu1 and mu2 and covariance matrices Sigma1, Sigma2.