bug#15618: 24.3.50; Region face takes priority over any overlay faces inside?

[Dmitry Gutov <dgutov@yandex.ru>]

I'm seeing inconsistent behavior with this.

There is an overlay inside the active region, it has a `face' property.
With default theme, the foreground and background attributes of that
face are never used, the values from `region' face are used.

But if I switch to another theme (e.g. M-x load-theme RET leuven), the
inner overlay becomes easily visible.

Backstory here: https://github.com/leoliu/easy-kill/issues/3

To reproduce:

1. Install `easy-mark' from https://github.com/leoliu/easy-kill.git
2. Open a buffer with some Elisp, move point to where some text is, type
`M-x easy-mark'.
3. Some part of the buffer becomes selected with an active region. The
previous position of point is marked by the "origin" overlay, with face
that's inheriting from `error'. However, the colors don't come through.

Here's how it looks for me: http://imgur.com/a/2VZyZ

Here's how it looks for Leo:
http://i.imgur.com/i8pxksI.png

This is also what I see (except for the exact colors) if I use one of
many other themes (such as leuven, adwaita, dichromacy).

Curiously, wheatgrass behaves the same as the default one in this
respect. And if I switch to wheatgrass and only then to leuven, then I
also see the same behavior there.

Which is the right behavior? Like I mentioned in the discussion there,
Displaying Faces[0] seems to indicate that `region' face should prevail
at all times.

[0] http://www.gnu.org/software/emacs/manual/html_node/elisp/Displaying-Faces.html

In GNU Emacs 24.3.50.3 (x86_64-unknown-linux-gnu, GTK+ Version 3.6.4)
 of 2013-10-15 on axl
Bzr revision: 114665 dgutov@yandex.ru-20131015012122-lo6jx0rfcpjoi6ls
Windowing system distributor `The X.Org Foundation', version 11.0.11303000
System Description:	Ubuntu 13.04