Make toolbar height compatible with previous wx version

Restore the hack with making the toolbars with wxTB_FLAT style 3px less
tall than the value given to them by TB_AUTOSIZE.

There is no real explanation for doing this, this adjustment was added
by 98b9643647 back in 2002 with the
rationale "make flat toolbars look slightly better", but after doing it
for 17 years during which nobody complained about this, we probably
should keep doing it now.

src/msw/toolbar.cpp

@@ -1678,6 +1678,8 @@ void wxToolBar::UpdateSize()
     wxPoint pos = GetPosition();
     ::SendMessage(GetHwnd(), TB_AUTOSIZE, 0, 0);
 
+    wxSize size = GetSize();
+
     // TB_AUTOSIZE doesn't seem to work for vertical toolbars which it resizes
     // to have the same width as a horizontal toolbar would have, even though
     // we do use TBSTATE_WRAP which should make it start a new row after each
@@ -1708,11 +1710,19 @@ void wxToolBar::UpdateSize()
                 maxWidth = width;
         }
 
-        SetSize(maxWidth, GetSize().y);
+        size.x = maxWidth;
+    }
+    else // horizontal
+    {
+        // For (flat) horizontal toolbars we used to make the toolbar 3px less
+        // tall in the previous wx versions, for some reason. It's not obvious
+        // at all if this is the right thing to do, but continue doing it now
+        // for compatibility.
+        if ( HasFlag(wxTB_FLAT) )
+            size.y -= 3;
     }
 
-    if (pos != GetPosition())
-        Move(pos);
+    SetSize(wxRect(pos, size));
 
     // In case Realize is called after the initial display (IOW the programmer
     // may have rebuilt the toolbar) give the frame the option of resizing the