Prepare fonts for entries only when adding an entry, rather than
for every entry on every frame.

Re-prep fonts to reflow and recalculate entry height when the
window width changes. This causes significant performance impacts
when resizing the window with a large log.

* Refactored Entry font prep and max prefix tracking to its own method
* Added a map for tracking prefix lengths
* When removing Entries, update max_prefix_width if necesarry
* Reset max_prefix_width (and tracking map) when resizing or clearing Entries

* Needs to account for draw_offset alignment kludge

…-tweak