Merge branch 'dc-clear-white-default'

Fix regression in wxDC::Clear() and make wxGCDC::Clear() consistent with
it by using white if the background brush hadn't been explicitly set.

See https://github.com/wxWidgets/wxWidgets/pull/1582

interface/wx/dc.h

@@ -261,6 +261,9 @@ public:
         Note that SetBackground() method must be used to set the brush used by
         Clear(), the brush used for filling the shapes set by SetBrush() is
         ignored by it.
+
+        If no background brush was set, solid white brush is used to clear the
+        device context.
     */
     void Clear();