win32: Report ScrollDC update region directly

win32: Report ScrollDC update region directly

Rather than set the window update region and repaint this region
when we get a WM_PAINT we just directly add it to the update
region. No need to roundtrip via win32.

This lets us also make sure we do this drawing in the same update
cycle. This seems especially important on Win7, because ScrollDC
seems to act kind of weird there, not using bitblt in areas where
it seemingly could, which makes scrolling look really flashy.

http://bugzilla.gnome.org/show_bug-cgi?id=674051