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/controls/gridtest.cpp

@@ -122,10 +122,7 @@ GridTestCase::GridTestCase()
     m_grid->Refresh();
     m_grid->Update();
 
-    if ( !waitForPaint.YieldUntilPainted() )
-    {
-        WARN("Grid not repainted until timeout expiration");
-    }
+    waitForPaint.YieldUntilPainted();
 }
 
 GridTestCase::~GridTestCase()