bug#16969: Isearch: lazy-highlight face sometimes has foreground and background the same colour.

[Drew Adams <drew.adams@oracle.com>]

> We need to guarantee that default faces don't produce
> illegible combinations of foreground and background colors.

Yes, that is the _only_ *need* here.  It is the need raised
by the bug.

And the simplest and most foolproof way to fulfill that need and
fix the bug is to simply give face `lazy-highlight', by default,
a black/white foreground for a light/dark background mode.

Just as we do already for face `isearch', and just as we have
always done.  That should have been done for `lazy-highlight'
long ago.  End of story.

Any user who wants to fiddle with the new "feature" that lets
other highlighting show through and tries to adjust colors
automatically can always customize these search-hit faces to get
that new effect.  There is no reason to impose this gimmick on
Emacs users now by _default_.

Today's flashy new feature is too often tomorrow's out-of-fashion
annoyance.  Just stick with what is simple and has always worked
well.  If, after a few years, we find that _most users choose_ to
customize to get the new effect, we can then turn it on by default.
There is no reason to jump to that now.

It amazes me that it took so long and was such a bloody battle to
get something like `transient-mark-mode' turned on by default,
even though users commonly customized Emacs for decades to turn it
on, and yet you are ready to willy nilly change longstanding
default behavior such as this before the new behavior has even
been offered as a possibility for users to _choose_.  It hasn't
even been released yet, and you already want to make it the default.

First things first: turn on `delete-selection-mode' by default,
why dontcha?  And offer this new highlighting style as an
_option_ for now: opt in, not opt out, for the new.

Consider making it the default behavior only later, after you have
had a chance to see how many users actually choose it.  Don't get
carried away by your enthusiasm for your shiny new object.