I have updated my patch a bit:

1. Instead of setting the new API to NULL when the scale factor is
ignored or unsupported by the backend, I'm setting it to a dummy
function returning 1. This keeps backwards compatibility and is more
convenient for the user of the interface.

2. get_scale_factor now returns a double. Even though current scale
factors are integers, this is an interface that will better have to
make it through the forthcoming era of fractional scaling.