user32: Enforce that surfaces are flushed after ReleaseDC.

[Sebastian Lackner]

If a second thread draws on the surfaces while the main thread is waiting for
window messages the window doesn't update properly. Can be reproduced for example
with the source code from: https://www-user.tu-chemnitz.de/~heha/petzold/ch20c.htm

---
 dlls/user32/painting.c | 1 +
 dlls/user32/win.c      | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)
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