Rework the rendering model so that clients instantiate backing stores.

This makes interactive resizing work a lot better, althought it's still not
perfect. There are still glitches and unpleasant flashes of zeroed memory.

Applications/Terminal/Terminal.cpp

@@ -760,3 +760,9 @@ void Terminal::force_repaint()
     m_need_full_flush = true;
     update();
 }
+
+void Terminal::resize_event(GResizeEvent&)
+{
+    m_needs_background_fill = true;
+    force_repaint();
+}