Fix background painting of multi-row toolbars in wxMSW.

The item rectangle used wrong vertical coordinates which didn't matter (much?)
for the single row toolbars but was completely wrong for toolbars with more
than one row.

Don't always anchor the rectangle at the window top to avoid this.

See #13579.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76105 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/toolbar.cpp

@@ -1886,8 +1886,7 @@ bool wxToolBar::HandlePaint(WXWPARAM wParam, WXLPARAM lParam)
                 }
                 else
                 {
-                    rcItem.top = 0;
-                    rcItem.bottom = rectTotal.height;
+                    rcItem.bottom = rcItem.top + rectTotal.height / m_maxRows;
                 }
 
                 rgnDummySeps.Union(wxRectFromRECT(rcItem));