Hinted metrics fixes

This is my attempt at fixing the issue identified in #626 (closed) as well as the potential issue with the custom ceil code also described in that issue. I also updated the test_empty_line_height in a way that would have caught the issue from the main fix of this MR.

Please be extra careful when reviewing since all of this is based on my at best rudimentary understanding of Pango and the entire font stack.