Fix general_math_ev3 so that it doesn't escape % when it is already escaped.

The `general_math_ev3` function manipulates the TeX string that it receives, and (among other things), it puts backslashes in front of percent signs.  This "fixes up" bad TeX entered by problem authors, but it also messes up good TeX created by MathObjects.

This patch prevents the escaping of `%` when it already is escaped (i.e. is preceded by an even, possibly 0, number of backslashes).  It also fixes a similar problem with backslashes at the end of the string.

A test case for this is:

```
DOCUMENT();

loadMacros(
  "PGstandard.pl",
);

BEGIN_TEXT
\(x\% x\)
END_TEXT

ENDDOCUMENT();
```

Without the patch, it will produce a math-mode x by itself, and with the patch, it should be the three characters `x%x` in math mode.  I could not find a decent way to test the trailing slash fix, because the extra space is ignored in math mode when it is not part of the preceding macro.  So I guess that change could be eliminated if you want, but there is no need for the extra space when there are an even number of backslashes at the end.

My own feeling is that the code that puts out the math for the current displayMode should be factored out so that it can be called *without* having such modifications made to it.  Math that is already properly formatted may be damaged (as in the case of the percent), and currently there is no way to avoid this (other than to modify the math so that it doesn't match these patterns).  There should be a way to get math output that is unmodified.