With the 3.0 transition, this code went from just querying the entry's
height request to doing a full size request.
Then it got code to revert the features that a full size request does.
And then it grew code that manually computed the baseline.

Avoid this and just do what happened back in the days: Do a regular
height request.

This changes the semantics of the get_frame_size() vfunc wrt its
behavior towards subclasses overwriting the get_height() vfuncs, but I'm
happy to live with that.