Fix backend arguments in concrete minkowski sum

[mforets]

This PR passes the backend argument to vertices_list when needed which would be otherwise ignored.

There is also a fix in the sizehint.

[schillic]

I would have expected that you do

function _vertices_list(P::AbstractPolytope, backend)
    return vertices_list(P)
end
function _vertices_list(P::HPolytope, backend)
    return vertices_list(P, backend=backend)
end

so it also affects the first method with AbstractPolytope (which might be an HPolytope).