[v3] riched20: allow setting background text color

[Huw Davies]

On Thu, Aug 11, 2016 at 12:22:21PM -0600, Kenneth Haley wrote:
> fixes https://bugs.winehq.org/show_bug.cgi?id=36625
> 
> Tested on Arch Linux.

Now the background is correctly painted, but there's
still the issue of the background disappearing when
selecting a range within a run of text that has a
background.

It's easy to test this using Wine's wordpad.

Huw.