Inplace multiplication by a square matrix

Not without doing something even more convoluted, as far as I can tell.

You could just do

@views for i = 1:size(X, 2)
    X[:,i] = R * X[:,i]
end

in 0.6 with the new @views macro, but you need to call A_mul_B! if you want to do the matrix*vector with a preallocated output vector.