[PATCH 2/2] riched20: Only call InvalidateRect if the background color changes.

[Alex Henrie]

Please ignore this patch. After sending it, I noticed that the
backgrounds in Word Viewer changed from gray to white after leaving
the tab and returning to it, when on Windows they stay gray the whole
time. More tests are needed to determine when exactly InvalidateRect
should be called.

The first patch is still good, and does not appear to cause any
negative side effects.

-Alex