assdraw: fix assertion failure from reading and writing a bitmap at the same time

Originally committed to SVN as r6860.

contrib/assdraw/src/wxAGG/AGGWindow.cpp

@@ -28,11 +28,12 @@ namespace GUI {
         if (ncheight < 1) ncheight = 1;
 
         bitmap = new wxBitmap(ncwidth, ncheight, PixelFormat::wxWidgetsType::BitsPerPixel);
-        memDC.SelectObject(*bitmap);
 
         // Draw the bitmap
         attachAndDraw();
 
+        memDC.SelectObject(*bitmap);
+
         // Request a full redraw of the window
         Refresh(false);
     }