user32: invalidate button area instead of painting immediatly

[Clinton Stimpson]

WM_SETFONT on a button should invalidate the area so the system can send 
a WM_PAINT event, instead of painting the button immediately.
This is a partial fix for bug# 6006.
The other part of the fix is for ShowWindow(button, SW_HIDE) to cause a 
WM_PAINT event on the button's parent to paint in the newly exposed region.

Thanks,
Clinton Stimpson

ChangeLog:
    user32:  invalidate button area instead of painting button immediately.