Keep a cache of any records which needed converting to the ASCII
locale used for searching.

These changes would have to be broadened if somebody wanted to
search in a more complex locale; currently the only characters
allowable are [a-z0-9]. It would need a greater understanding of
how SDL handles the user input.

We have limited options for text rendering, so must do a conversion
from the system locale to UTF-8 where necessary.

Deliberately not using TTF_RenderUNICODE and wchar_t conversion,
despite it being simpler;  because the system wchar_t is 32-bit,
but SDL's idea of unicode is 16 bit.

Leave iconv out of the path of the rendering that happens every
frame as these are known non-unicode strings.

Thanks to Stefan Berg-Johansen for a high quality bug report.

> The type declaration on line 79 in`player.c` should read `static
> unsigned short x = 0xbabe;`. In the current master branch, `x` is a
> static _signed_ short, which results in it taking on negative values
> outside of the intended range. Another side effect is that the
> returned sequence happens to converge after about 15 calls to a
> small negative number, so effectively the current dither function
> only imposes a small constant offset and nothing else.

This massively decreases the time required to build the lookup tables.

Previously we were looking for any function key in correspondence
with the Ctrl key.

Thanks for Robert Flechtner for repoting this bug.