Allow to not create wxPaintDC in EVT_PAINT handler in wxMSW.

Failure to create a wxPaintDC in EVT_PAINT handler resulted in many serious
and difficult to debug problems under wxMSW. We used to document that the user
shouldn't do it but this wasn't enough (see #11648). We could also assert if
we detected that a handler didn't create a wxPaintDC but it seems better to
just handle this case gracefully for consistency with the other platforms.

Add wxDidCreatePaintDC global variable which is reset before generating
wxPaintEvent and set to true when ::BeginPaint() is called from wxPaintDC
ctor and validate the update region of the window ourselves if it wasn't set
(meaning that wxPaintDC wasn't created).

Update the documentation to emphasize the link between EVT_PAINT handlers and
wxPaintDC but without saying that wxPaintDC object must always be created in
the handler as this is not true any more.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63229 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/event.h

@@ -1547,16 +1547,8 @@ public:
 
     A paint event is sent when a window's contents needs to be repainted.
 
-    Please notice that in general it is impossible to change the drawing of a
-    standard control (such as wxButton) and so you shouldn't attempt to handle
-    paint events for them as even if it might work on some platforms, this is
-    inherently not portable and won't work everywhere.
-
-    @remarks
-    Note that in a paint event handler, the application must always create a
-    wxPaintDC object, even if you do not use it. Otherwise, under MS Windows,
-    refreshing for this and other windows will go wrong.
-    For example:
+    The handler of this event must create a wxPaintDC object and use it for
+    painting the window contents. For example:
     @code
     void MyWindow::OnPaint(wxPaintEvent& event)
     {
@@ -1565,6 +1557,12 @@ public:
         DrawMyDocument(dc);
     }
     @endcode
+
+    Notice that you must @e not create other kinds of wxDC (e.g. wxClientDC or
+    wxWindowDC) in EVT_PAINT handlers and also don't create wxPaintDC outside
+    of this event handlers.
+
+
     You can optimize painting by retrieving the rectangles that have been damaged
     and only repainting these. The rectangles are in terms of the client area,
     and are unscrolled, so you will need to do some calculations using the current
@@ -1601,6 +1599,12 @@ public:
     }
     @endcode
 
+    @remarks
+    Please notice that in general it is impossible to change the drawing of a
+    standard control (such as wxButton) and so you shouldn't attempt to handle
+    paint events for them as even if it might work on some platforms, this is
+    inherently not portable and won't work everywhere.
+
 
     @beginEventTable{wxPaintEvent}
     @event{EVT_PAINT(func)}