Fix BUG:472073, use font-size for line-height ratio calculation.

This fixes BUG:472073,

first part fixes the actual line-height, second part fixes the empty line with a hardbreak getting acknowledged for line-height calculations, though the latter might need some testing (like, what happens in other software when the hardbreak is pt 32 and the rest of the line is 10pt? EDIT: This indeed uses the first non-collapsed glyph, will need to think how to do this).

Pushed this to the main repository because I wasn't paying attention...

Test Plan

Check the SVG at https://bugs.kde.org/show_bug.cgi?id=472073 in Krita.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
  • The unit-tests have the usual png render issues, but I cannot see actual breakage.
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.

Reminder: the reviewer is responsible for merging the patch, this is to ensure at the least two people can build the patch. In case a patch breaks the build, both the author and the reviewer should be contacted to fix the build. If this is not possible, the commits shall be reverted, and a notification with the reasoning and any relevant logs shall be sent to the mailing list, kimageshop@kde.org.