#1202 remove panic on unfreed fonts

[Don Porter]

Line 439 of Revision 1.13 of generic/tkFont.c is a
panic() when the font subsystem is finalizing, yet
there are unfreed fonts.

Unfortunately, we know that some Tk widgets, notably
[text] and [canvas], do not always free the fonts they
are using. This panic() has been helpful in uncovering
those bugs, but it is not a good thing to expose to
end users. We should either remove it, or perhaps
disable it unless a debugging version of Tk is compiled.

See also Tk Bugs 559450 and 458309.

[Don Porter]

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increasing priority, but not to 9. This issue is not
critical for
the beta release, but I believe it is for 8.4.0.

[Jeffrey Hobbs]

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changed to an assert and wrapped panic in #ifdef PURIFY for
8.4b1.