size-manager: Always return a suitable height

When we have multiple outputs and thus can't yet reliably determine the optimal height fall back to the default height rather than 0.

This frees the upper layers from guessing a suitable height (which they got wrong 😀).

Note that this doesn't make an attempt on fixing #68 . With this minimal fix we could try to squeeze it into 0.50.

Closes #75 (closed)