EXAMPLES/NX FONT CACHING

```
Description: The font caching logic in examples/nx is incomplete. Fonts are
added to the cache, but never removed. When the cache is full
it stops rendering. This is not a problem for the examples/nx
code because it uses so few fonts, but if the logic were
leveraged for more general purposes, it would be a problem.

Update: see examples/nxtext for some improved font cache handling.
Update: The NXTERM font cache has been generalized and is now
offered as the standard, common font cache for all applications.
both the nx and nxtext examples should be modified to use this
common font cache. See interfaces defined in nxfonts.h.
Status: Open
Priority: Low. This is not really a problem because examples/nx works
fine with its bogus font caching.
```