This makes it possible to write templated alternatives for ConvertibleThroughBase
which are then available for any (strong) type that implements a base() function
In the end making adding new ConvertibleThroughBase types less awkward.
That it works for the version we have packaged it pure coincidence, as that is
one of the few versions that due to a bug allow it. So add the appropriate
template specialisations to support it out-of-the-box within OpenTTD.