Simplify WaitForPaint::YieldUntilPainted()

Don't return anything from it, just warn directly -- this is what all
the existing callers did, so it's simpler if the function just does it
itself instead of forcing them to check its return value.

Also reset m_painted after yielding, so that YieldUntilPainted() could
be called again, if necessary.

tests/window/setsize.cpp

@@ -98,10 +98,7 @@ TEST_CASE("wxWindow::MovePreservesSize", "[window][size][move]")
 
     w->Show();
 
-    if ( !waitForPaint.YieldUntilPainted() )
-    {
-        WARN("Didn't get a paint event until timeout expiration");
-    }
+    waitForPaint.YieldUntilPainted();
 
     const wxRect rectOrig = w->GetRect();