First draft patch trying to fix [f214b8ad5b].
New test font-47.1 demonstrates effectiveness of the fix.
Since fontCache is the first field of struct TkFontInfo, make the code somewhat better readable. But actually it's no change at all.
This is my only review comment! Good work!
Add test-case for bug [f214b8ad5b], in order to prove that Tk 8.5 doesn't suffer.
Fix bug [f214b8ad5b]: shutdown woes